01 - Preheat 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 light and fluffy, about 3 minutes.
03 - Add the eggs one at a time, beating well after each addition. Mix in the vanilla extract until fully 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, cohesive dough forms. Avoid overmixing.
06 - Divide the dough equally into three separate bowls. To the first bowl, add the cocoa powder and mix thoroughly 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 colored 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 and smooth with an offset spatula. Carefully layer the vanilla dough over the chocolate, spreading evenly. Finish by layering the strawberry dough on top, smoothing the surface flat.
08 - Bake for 12 to 15 minutes until just set. Do not overbake—the layers should remain soft and slightly pale.
09 - Cool completely in the pan on a wire rack. Use the parchment overhang to lift the slab out, then cut into 36 bars or squares.
10 - Optionally, drizzle melted semisweet chocolate over the cut bars and decorate with sprinkles or chopped nuts. Allow the chocolate to set before serving.