01 - Preheat the oven to 350°F. Line a 9 x 13 inch baking pan with parchment paper, leaving an overhang on the sides for easy removal.
02 - In a large bowl, beat the softened butter and granulated sugar together using an electric mixer until pale, light, and fluffy, about 3 minutes.
03 - Add the eggs one at a time, beating well after each addition. Scrape down the sides of the bowl, then mix in the vanilla extract until combined.
04 - In a separate 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, mixing on low speed until a soft, uniform dough forms. Do not overmix.
06 - Divide the dough evenly into three bowls. To the first bowl, add the cocoa powder and mix until fully incorporated for the chocolate layer. To the second bowl, add the strawberry extract and a few drops of pink or red gel food coloring, blending 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 with an offset spatula. Carefully spread the vanilla dough over the chocolate layer, smoothing evenly. Finally, spread the strawberry dough on top, smoothing the surface flat.
08 - Bake for 12 to 15 minutes until just set and the edges are lightly golden. Do not overbake, as the layers should remain soft and tender.
09 - Let the slab cool completely in the pan on a wire rack. Once fully cooled, use the parchment overhang to lift it out. Cut into 36 bars or squares with a sharp knife.
10 - Optionally, drizzle melted semisweet chocolate over the cut bars and decorate with sprinkles or chopped nuts. Allow the chocolate to set before serving.