Ingredients
Equipment
Method
- Position the oven rack in the middle slot and preheat to 350°F (175°C), then grease an 18x13 inch (46x33cm) rimmed light-colored aluminum half-sheet pan thoroughly with softened butter or non-stick spray on the bottom and all four sides — do not line with parchment.
- In a large mixing bowl, whisk together 2 cups (240g) all-purpose flour, 2 cups (400g) granulated sugar, ¼ cup (25g) natural unsweetened cocoa powder, 1 tsp (6g) baking soda, and ½ tsp (3g) fine sea salt for 30 seconds until no white streaks remain and the mixture looks uniformly dark brown.
- In a medium saucepan over medium heat, combine 1 cup (226g) unsalted butter, 1 cup (240ml) whole milk, and ¼ cup (25g) natural unsweetened cocoa powder, whisking constantly for 4 to 6 minutes until the butter is fully melted and the mixture is completely smooth and glossy — remove from heat the moment you see small bubbles forming at the edges, do not boil.
- Pour the warm chocolate liquid slowly into the bowl of dry ingredients and whisk with wide strokes from the bottom upward for approximately 20 to 25 strokes until the batter is completely smooth and thin — this order cools the hot liquid through contact with room-temperature flour and sugar before the eggs go in, which prevents scrambling without the standard melt-and-mix cooling wait.
- Add 2 large room-temperature eggs and 1 tsp (5ml) pure vanilla extract to the batter and whisk just until fully incorporated — approximately 10 to 15 strokes — stopping as soon as no streaks remain; do not overmix.
- Pour the batter into the prepared pan, spread flat with a silicone spatula in 4 to 5 strokes, and bake on the middle rack at 350°F (175°C) for 20 to 25 minutes until the surface looks completely set and matte and the center springs back cleanly when pressed with one fingertip — start checking at 20 minutes.
- During the last 5 to 8 minutes of baking, make the icing: combine ½ cup (113g) unsalted butter, 6 tablespoons (90ml) whole milk, and 3 tablespoons (18g) natural unsweetened cocoa powder in the same saucepan over medium heat, whisking constantly for 3 to 4 minutes until smooth and glossy, then remove from heat, add 3½ cups (420g) sifted powdered sugar and 1 tsp (5ml) pure vanilla extract, and whisk vigorously for 30 to 45 seconds until completely smooth and thick but still pourable.
- The moment the cake comes out of the oven, place the pan on a wire cooling rack and immediately pour the warm icing over the hot cake surface, spreading gently with an offset spatula in 6 to 8 slow strokes from the center outward until the icing reaches all four edges.
- If using nuts, immediately scatter ½ cup (55g) roughly chopped toasted pecans or walnuts over the warm icing and press in very gently to anchor — if using flaked sea salt, scatter approximately ½ tsp (2g) Maldon-style flakes across the surface at the same time.
- Allow the cake to cool at room temperature for a minimum of 1 hour until the icing is fully set — the carryover heat from the cake continues setting the crumb during the first 15 to 20 minutes, so do not cut early — then slice into 20 even squares by scoring 4 cuts along the long side and 3 cuts along the short side, wiping the blade clean with a damp towel after every cut.
Notes
- Cocoa powder: use natural unsweetened cocoa powder only, checked for freshness — not Dutch-process. Dutch-process has neutralized acidity and will not activate the baking soda, producing a flat, dense cake. Open the container and smell it — fresh cocoa smells deeply chocolatey. Flat or dusty smell means replace it.
- Baking soda: test freshness before baking — drop ½ tsp into hot water with a splash of vinegar. It must bubble aggressively within 2 seconds. Replace if weak.
- Powdered sugar: sift before measuring and adding to the icing. Unsifted powdered sugar causes permanently grainy icing that cannot be fixed after it sets.
- Eggs: must be at room temperature — pull from fridge 30 minutes before starting, or warm in a bowl of warm water for 10 minutes.
- Nuts: toast raw pecans or walnuts in a dry skillet for 3 to 4 minutes before chopping — raw nuts taste flat against the icing. Chop to approximately ¼ inch (6mm) pieces.
- Flaked sea salt: optional but visible in the photos — use Maldon-style flakes only. Fine salt dissolves into the warm icing and will not be visible or add texture.
- Pan: use an 18x13 inch (46x33cm) light-colored aluminum half-sheet pan — dark pans overbrown the bottom. If using glass or ceramic, reduce oven temperature to 325°F (165°C) and increase bake time by 5 to 8 minutes. No parchment needed.
- Storage: cover loosely with foil directly over the pan — do not press the foil against the icing surface. Store at room temperature up to 4 days. Best on day two.
- Freeze baked: slice into squares, wrap each individually in plastic wrap, freeze up to 2 months. Thaw at room temperature 45 minutes to 1 hour — do not microwave.
- Freeze unbaked: do not freeze or refrigerate the batter. The baking soda reacts with the cocoa the moment the warm liquid hits the dry ingredients — by the time frozen batter thaws, the leavening has spent itself and the cake will bake flat. Bake immediately after mixing.
- Make ahead: bake the cake fully, ice it, and store covered at room temperature. The cake is best on day two — the crumb settles and the chocolate flavor deepens overnight. Do not make the batter ahead.
- Dairy-free: replace butter with an equal weight of Miyoko's or Country Crock Plant Butter sticks — not spreadable tub margarine. Replace whole milk with full-fat canned coconut milk. The icing sets slightly firmer with dairy-free butter — pour it a little warmer and spread quickly.
- Scaling: to halve the recipe, use a 9x13 inch (23x33cm) pan and bake 30 to 35 minutes. Reduce baking soda to ½ tsp — do not scale leavening linearly or the cake will taste soapy.
- Altitude: above 3,500 feet (1,067m), reduce baking soda to ¾ tsp, add 2 tablespoons (16g) flour, reduce sugar by 2 tablespoons (25g), increase oven to 365°F (185°C), check at 18 minutes.
- Allergens: contains gluten, dairy, and eggs. Contains tree nuts if pecans or walnuts are used.
- Tip: pre-measure all icing ingredients before the batter goes into the oven — the icing must be made and poured the moment the cake comes out, with no time to measure mid-process.