01 - Preheat oven to 350°F. Line a 9×13-inch baking pan with parchment paper, leaving an overhang on the sides for easy removal.
02 - In a large bowl, beat softened butter and sugar together using an electric mixer until pale, light, and fluffy, about 3 minutes.
03 - Add eggs one at a time, beating well after each addition. Stir in vanilla extract until evenly combined.
04 - In a separate bowl, whisk together all-purpose flour, baking powder, and salt until evenly distributed.
05 - Gradually add the flour mixture to the butter mixture, mixing on low speed until a soft, cohesive dough forms. Avoid overmixing.
06 - Split the dough evenly into three bowls. To the first bowl, fold in cocoa powder until fully blended for the chocolate layer. To the second bowl, add strawberry extract and a few drops of pink or red gel food coloring, mixing until uniformly tinted for the strawberry layer. Leave the third bowl plain for the vanilla layer.
07 - Press the chocolate dough evenly across the bottom of the prepared pan, smoothing with an offset spatula. Carefully spread the vanilla dough over the chocolate layer. Finish by pressing the strawberry dough on top, smoothing the surface flat.
08 - Bake for 12 to 15 minutes until just set. Do not overbake—the top should feel firm but not dry.
09 - Let cool completely in the pan on a wire rack. Lift the slab out using the parchment overhang, then cut into 36 bars or squares. Optionally drizzle with melted chocolate and top with sprinkles or chopped nuts before serving.