In a large mixing bowl, cream together butter and sugar using an electric mixer on medium speed for about 2-3 minutes until light and fluffy.

Beat in egg and vanilla extract to the creamed mixture and mix on low speed until fully incorporated.

In a separate bowl, whisk together flour, baking powder, and salt. Gradually add this to the wet mixture and mix on low until just combined.

Divide dough into two portions. To one half, mix in cocoa powder until combined.

Preheat oven to 350°F (175°C) and line a baking sheet with parchment paper. Scoop alternating spoonfuls of the chocolate and vanilla dough onto the sheet.

Using a toothpick, swirl the dough in a figure-eight motion to create the zebra pattern.

Bake for 10-12 minutes until the edges are golden and centers are set. Remove and let cool.

Transfer to a wire rack to cool completely before serving.