In a large mixing bowl, cream together the butter and sugar until light and fluffy. Beat in the egg and vanilla until well combined. In another bowl, whisk together flour, baking powder, and salt. Gradually add the dry ingredients to the wet mixture until a soft dough forms.

Divide the dough evenly into three bowls.

For the vanilla layer, leave one part plain.

For the chocolate layer, mix in the cocoa powder.

For the strawberry layer, add strawberry extract and food coloring. If any dough becomes too dry, mix in a teaspoon of milk to soften.

Line a loaf pan with plastic wrap. Press the strawberry dough evenly into the bottom of the pan. Next, press the vanilla dough on top of that, followed by the chocolate layer. Cover and chill for at least 1 hour or until firm enough to slice.

Preheat oven to 350°F (175°C). Remove the dough block from the loaf pan and cut into ¼-inch thick slices. Place on parchment-lined baking sheets about 2 inches apart. Bake for 10–12 minutes, or until edges are just set. Let cool on the pan for 5 minutes, then transfer to a wire rack.