In a medium bowl, whisk together 2¼ cups (270g) all-purpose flour (spooned and leveled), 1 tsp (4g) baking soda, and ¾ tsp (4.5g) fine sea salt for 20 full seconds until no clumps remain, then set aside.

In the bowl of a stand mixer fitted with the paddle attachment, beat 1 cup (226g) unsalted butter softened to 65–68°F, ¾ cup (165g) packed light brown sugar, and ¼ cup (50g) plain white granulated sugar on medium speed for exactly 3 minutes until the mixture is pale, fluffy, and noticeably increased in volume.

With the mixer on low speed, add 1 large room-temperature egg and beat for 20 seconds, then add 1 large room-temperature egg yolk and beat for 20 seconds more, then add 2 tsp (10ml) pure vanilla extract and beat for 10 seconds until fully incorporated and the batter looks smooth and glossy.

Remove the bowl from the mixer and add the entire dry ingredient mixture at once, then fold with a silicone spatula using wide bottom-upward strokes, counting 20 strokes and stopping the moment no dry flour streaks remain — do not use the mixer for this step.

Add 1½ cups (255g) semi-sweet chocolate chips and fold with the same spatula for approximately 10 strokes until evenly distributed, reserving the remaining ¼ cup (45g) chips for the top.

Transfer the dough to the prepared pan and press into an even layer reaching all the way to the edges using your fingers or the flat bottom of a measuring cup, then scatter the reserved ¼ cup (45g) chips across the top surface and press each one gently into the dough so they are anchored but still sitting proud.

Bake on the middle rack at 350°F (175°C) for 22–25 minutes until the edges are deep golden brown and fully set and a toothpick inserted at the true center comes out with a few moist crumbs — not wet batter, not completely clean — then remove from the oven immediately.

Place the pan on a wire cooling rack and cool in the pan for exactly 30 minutes before unmolding or slicing — do not cut before 30 minutes or the center will still be molten underneath the set surface.

To make the optional chocolate buttercream, beat ½ cup (113g) softened unsalted butter on medium speed for 2 minutes, then add 1½ cups (180g) sifted powdered sugar and 3 tbsp (18g) sifted cocoa powder and mix on low for 30 seconds before increasing to medium for 1 minute, then add 2–3 tbsp (30–45ml) heavy cream one tablespoon at a time and beat until the buttercream holds a stiff peak for 5 seconds when the beater is lifted.

Once the cookie cake is fully cool (at least 1 hour from the oven), transfer buttercream to a piping bag fitted with a Wilton 1M star tip and pipe a shell border around the perimeter, then scatter rainbow sprinkles across the center surface immediately before the buttercream crusts.