Preheat your oven to 350°F (175°C). Grease and flour a 9-inch round cake pan or a 9×9-inch square pan.

1. In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
2. In a large bowl, beat the softened butter and granulated sugar together until light and fluffy, about 3-4 minutes.
3. Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
4. Gradually add the dry ingredients to the butter mixture, alternating with the milk. Begin and end with the dry ingredients. Mix until just combined.
5. In a small bowl, combine the cocoa powder and boiling water. Stir until smooth.
6. Divide the batter in half. Add the chocolate mixture to one half and stir until fully incorporated.
7. Spoon alternating spoonfuls of the vanilla and chocolate batter into the prepared pan. Use a knife or skewer to gently swirl the batters together to create the marbled effect.
8. Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
9. Cool the cake in the pan for 10 minutes before transferring to a wire rack to cool completely. Slice and enjoy!