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 the softened butter and sugar together using an electric mixer until pale, light, and fluffy.
03 - Add the eggs one at a time, beating well after each addition. Mix in the vanilla extract until fully blended.
04 - In a separate bowl, whisk together the flour, baking powder, and salt. Gradually add the dry mixture to the butter mixture, mixing on low speed until a soft, uniform dough forms.
05 - Split the dough evenly into three separate bowls. To the first bowl, fold in the cocoa powder until thoroughly combined 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.
06 - 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 to an even thickness. Finally, press the strawberry dough over the vanilla layer and smooth the top surface.
07 - Bake for 12 to 15 minutes, or until just set. Avoid overbaking to keep the layers soft and tender.
08 - Allow to cool completely in the pan on a wire rack. Lift the slab out using the parchment overhang and cut into bars or squares. Optionally, drizzle with melted semisweet chocolate and decorate with sprinkles or chopped nuts.