First prepare your cake pans with grease & parchment paper lining & preheat the oven to 350°F
In a medium sized bowl combine the cocos powder with the oil & hot water & whisk smooth
Prepare the cake recipe as per the instructions on that recipe post & then add a scant 1 cup of the vanilla batter to the cocoa mixture & whisk smooth
Now divide the vanilla batter evenly amongst your prepared cake pans alternating with a small amount of the chocolate batter & then swirl the 2 batters together with a skewer or a small spatula
Bake for 25-35 minutes depending on your pans size~ until a toothpick inserted comes out with moist crumbs
Cool in the pans until you can safely touch them then flip the cake layers out onto a cooling rack to cool to cold
Meanwhile prepare the buttercream recipe of your choice according to the instructions on that recipe post
Prepare the cocoa paste mixture by combining the cocoa with the hot water & whisk smooth
Add about 1 cup of vanilla buttercream to the COLD cocoa paste & mix smooth
Assemble to cake as per the video
Notes
Storage Marble cake can be kept at room temperature for several days in a cool dry climate.. For longer storage refrigerate for up to 10 days wrapped loosely to prevent drying of the cut slices. Freeze the cake layers wrapped well for up to 1 month