01 - Preheat oven to 350°F. Line a 9 x 13 inch baking pan with parchment paper, leaving an overhang on two sides for easy removal.
02 - In a large mixing bowl, beat the softened butter and granulated sugar together using an electric mixer on medium speed until pale, light, and fluffy, about 3 to 4 minutes.
03 - Add the eggs one at a time, beating well after each addition until fully incorporated. Scrape down the sides of the bowl, then mix in the vanilla extract.
04 - In a separate medium bowl, whisk together the all-purpose flour, baking powder, and fine salt until evenly distributed.
05 - Gradually add the flour mixture to the butter mixture on low speed, mixing just until a soft, cohesive dough forms. Avoid overmixing.
06 - Divide the dough evenly into three separate bowls. To the first bowl, add the cocoa powder and mix until uniformly blended for the chocolate layer. To the second bowl, add the strawberry extract and a few drops of pink or red gel food coloring, mixing until evenly tinted for the strawberry layer. Leave the third bowl plain for the vanilla layer.
07 - Press the chocolate dough evenly into the bottom of the prepared pan, smoothing the surface with an offset spatula. Carefully spread the vanilla dough over the chocolate layer, smoothing gently. Finally, press the strawberry dough on top, smoothing to an even finish.
08 - Bake for 12 to 15 minutes, or until the top is just set and the edges are lightly golden. Do not overbake to maintain a tender texture.
09 - Allow to cool completely in the pan on a wire rack. Lift the slab out using the parchment overhang and cut into 36 bars or squares. If desired, drizzle with melted semisweet chocolate and decorate with sprinkles or chopped nuts.