Preheat the oven to 180°C (350°F) and prepare a bundt cake tin by greasing it with butter and dusting it with flour.
Add the softened butter and sugar to a bowl of a stand mixer fitted with a paddle attachment. Beat at medium-high speed until creamed, about 3 minutes.
With the mixer running at medium speed, add the eggs to the creamed butter and sugar mixture one at a time, scraping down the sides of the bowl with a rubber spatula between each addition. Add the vanilla and beat again.
Sift together the flour, baking powder, and salt into a large bowl.
Add the dry ingredients and sour cream to the cake batter by alternating between the two. Starting with the flour mixture and ending with the sour cream. Scrape down the sides of the bowl and mix again. Divide the cake batter in half and transfer half of the batter to a separate bowl.
Mix together the cocoa powder and hot water, and stir until the cocoa powder has dissolved. Add this cocoa mixture to one of the bowls of batter and use a whisk to combine. You now have one bowl of plain batter and one bowl of chocolate cake batter.
Use two ice cream scoops or two dessert spoons to spoon the different batters into the prepared Bundt cake tin. Do this by alternating between the two so that you have dollops of vanilla and chocolate batter filling the tin to create a marbled effect.
Bake the cake in a preheated oven for 45 minutes or until it is golden brown and a cake tester inserted into the center of the cake comes out clean. Allow the cake to cool in its tin for 20 minutes before turning it onto a wire rack to cool completely. Once cool, transfer it onto a cake plate.
Chocolate Drip
Add the chopped chocolate and evaporated milk to a small saucepan set over medium heat. Stir occasionally until the chocolate has melted.
Drizzle the melted chocolate sauce over the cooled marble cake. Slice, serve, and enjoy!
Notes
Ensure the cake has cooled before pouring over the chocolate drip, if the cake is too warm then the sauce will run down the sides of the cake rather than creating defined drips.