Preheat your oven to 350°F (175°C). Grease and flour a 9x13-inch baking pan.

In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy, about 3-4 minutes.

Add the eggs one at a time, mixing well after each addition, then stir in the vanilla extract.

In another bowl, whisk together the flour, baking powder, and salt. Gradually add this dry mixture to the butter mixture, alternating with the milk, starting and ending with the flour mixture. Mix until just combined.

In a small bowl, combine the cocoa powder and hot water to create a smooth chocolate mixture.

Scoop half of the batter into a separate bowl and mix in the chocolate mixture until well combined.

In the prepared baking pan, drop spoonfuls of the vanilla batter and chocolate batter alternately. Use a knife or skewer to gently swirl the batters together to create a marbled effect.

Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted into the center comes out clean. Allow the cake to cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.