Ingredients
Equipment
Method
- Position oven rack in the middle and preheat to 350°F (175°C), then line a 9x13 inch (23x33cm) light-colored aluminum pan with parchment paper leaving 2 inches (5cm) overhang on both long sides and lightly spray with non-stick spray.
- In a large bowl, combine the devil's food cake mix, 3 room-temperature eggs, ½ cup (109g) neutral oil, and 1 cup (237ml) water, then stir with a silicone spatula for 40–50 strokes until no dry streaks remain and the batter looks smooth and glossy.
- Pour the batter into the prepared pan and spread to the edges with the spatula, then set aside — do not bake yet.
- In a second large bowl, combine the 1 cup (258g) creamy commercial peanut butter, 4 oz (113g) softened full-fat block cream cheese, ½ cup (60g) sifted powdered sugar, and 2 tablespoons (30ml) heavy cream, then stir firmly for 30–40 strokes until completely smooth with no white cream cheese streaks remaining.
- Spoon the peanut butter filling onto the unbaked batter and spread evenly to the edges using an offset spatula or the back of a spoon — use the full quantity, the layer should be visibly thick across the entire surface.
- Bake on the middle rack for 40–45 minutes until the edges pull away from the pan by ¼ inch (6mm), the top looks matte and set, and a toothpick inserted into the chocolate base at a corner comes out with moist crumbs not wet batter.
- Remove from oven and cool on a wire rack for a minimum of 60 minutes until the surface feels at most very slightly warm before proceeding.
- In a small saucepan over medium-low heat, warm ½ cup (120ml) heavy cream until small bubbles appear at the edges — do not boil — then remove from heat, add 1 cup (170g) semi-sweet chocolate chips, let sit undisturbed for 2 minutes, then stir from the center outward until smooth and glossy.
- Allow ganache to cool for 5 minutes, then pour over the cooled cake in a slow back-and-forth drizzle across the full surface, letting it cascade over the sides.
- Immediately scatter the roughly chopped Reese's cups and roasted salted peanuts across the ganache surface, pressing larger pieces gently to anchor them, then allow to cool at room temperature for a minimum of 2 hours before cutting.
- For clean cuts, refrigerate the pan for 30 minutes after the 2-hour rest, then lift out using the parchment overhang, cut into 15 bars with a sharp knife wiped clean between every cut, and serve at cool room temperature.
Notes
- Peanut butter: commercial creamy only (Jif or Skippy) — natural peanut butter separates during baking and destroys the middle layer.
- Cream cheese: full-fat block only, not low-fat or whipped — must be fully softened or filling will have unmixable lumps.
- Cake mix: standard devil's food 15.25 oz — do not use pudding-added variety, it changes the base texture.
- Ganache: heat cream to edge-bubbles only, not boiling — boiling breaks the ganache.
- Storage: airtight hard-sided container, room temperature 2 days or refrigerated up to 5 days.
- Freeze: freeze cut bars individually 1 hour on parchment tray, then transfer to freezer container up to 2 months — thaw overnight in fridge.
- Make ahead: bake the day before, refrigerate overnight — the peanut butter layer firms and improves on Day 2.
- Scaling: to double, bake in two separate 9x13 pans — do not bake a doubled batch in one deeper pan.
- Altitude: follow the high-altitude instructions on the back of your cake mix box for the base.
- Allergens: contains peanuts, tree nuts (check Reese's label), eggs, dairy, wheat.
- No mixer required: this recipe comes together with a bowl and spatula only.
- No chilling required: bars go from mixing bowl to oven with no rest time needed.
- Tip: refrigerate 30 minutes after cooling before cutting — firms the ganache and peanut butter layer for clean cross-section cuts.