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 3-5 minutes).

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

In a separate bowl, whisk together the flour, baking powder, and salt. Gradually add this dry mixture to the wet ingredients, alternating with the milk. Mix until just combined.

In a small bowl, combine the cocoa powder and hot water to create a smooth chocolate mixture. Take about one cup of the cake batter and mix it with the chocolate mixture until well combined.

Spoon alternating dollops of the plain batter and the chocolate batter into the prepared baking pan. Use a knife or skewer to swirl the batters together gently for 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.