9x13 Marble Cake with Buttermilk

Instructions:

1. Preheat the oven to 350°F (175°C) and thoroughly grease a 9x13 inch baking pan.
2. Cream the softened butter and granulated sugar together using a mixer until the mixture is pale and fluffy.
3. Beat in the eggs one at a time, ensuring each is fully incorporated, then stir in the vanilla extract.
4. Whisk together the flour, baking powder, and salt in a separate bowl.
5. Gradually add the dry ingredients to the butter mixture, alternating with the buttermilk, starting and ending with the flour mixture.
6. Divide the batter equally between two mixing bowls.
7. In one bowl, whisk the cocoa powder and hot water into a paste, then fold it into the batter until deep brown.
8. Pour the vanilla batter into the prepared pan and smooth the top.
9. Spoon the chocolate batter in dollops over the vanilla layer. Use a knife or skewer to drag through the batter in a figure-eight motion to create the marble effect.
10. Bake for 35 minutes or until a toothpick inserted into the center comes out clean.
11. Prepare the buttercream by creaming the softened butter, then beating in powdered sugar, cocoa powder, heavy cream, and vanilla extract until smooth.
12. Spread the buttercream evenly over the cooled cake before slicing into 12 portions.