Directions:

Preheat your oven to 350°F (175°C). Grease and flour a 9-inch round or loaf pan.
In a large mixing bowl, beat together the softened butter and sugar until the mixture becomes light and fluffy.
Add in the eggs, one at a time, beating well after each addition. Stir in the vanilla extract.
In a separate bowl, whisk together the flour, baking powder, and salt.
Gradually add the dry ingredients into the butter mixture, alternating with milk, starting and ending with the dry ingredients. Mix until just combined.
In a small bowl, combine the cocoa powder with the hot water to make a smooth chocolate paste.
Take half of the batter and mix it with the chocolate paste, stirring until fully incorporated.
Spoon alternating layers of vanilla and chocolate batters into the prepared pan. Use a knife or skewer to swirl the batters together to create the marble effect.