• Step 1: Preheat your oven to 350°F (175°C). Grease and flour a 9-inch round baking pan.
• Step 2: In a medium bowl, whisk together the all-purpose flour, baking powder, and salt. Set aside.
• Step 3: In a large bowl, cream the unsalted butter and granulated sugar together until light and fluffy, about 2-3 minutes.
• Step 4: Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
• Step 5: Alternately add the flour mixture and milk to the butter mixture, mixing just until combined after each addition.
• Step 6: Divide the batter evenly into two separate bowls. In one bowl, stir in the unsweetened cocoa powder until well combined.
• Step 7: Spoon the chocolate batter and the plain batter into the prepared pan, alternating spoonfuls to create the zebra pattern.
• Step 8: Bake the cake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
• Step 9: Allow the cake to cool in the pan for 10 minutes, then transfer it to a wire rack to cool completely.