In a small bowl mix hot water, cocoa powder, and coffee powder until smooth. Set aside.

Combine buttermilk and vegetable oil in another bowl.

Beat butter and sugar until pale and fluffy.

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

Sift flour, salt, baking powder, and baking soda into the batter. Mix until just combined.

Add the buttermilk mixture in 3–4 additions, mixing gently after each addition.

Transfer one-third of the batter to another bowl and mix with the cocoa mixture.

Alternate spoonfuls of vanilla and chocolate batter into each pan. Swirl lightly with a knife.

Bake for 25–30 minutes or until a skewer inserted comes out clean.

Cool cakes for 10 minutes in the pans, then transfer to a wire rack to cool completely.

For milk chocolate ganache, melt milk chocolate, whipping cream, cocoa powder, and salt. Cool to room temperature.

For white chocolate ganache frosting, melt white chocolate with cream and salt. Cover and refrigerate for 1 hour, then whip until fluffy.

Assemble cake by spreading white chocolate ganache frosting on each layer and adding spoonfuls of milk chocolate ganache. Swirl gently.

Apply a thin crumb coat with white chocolate ganache frosting and chill 20 minutes.

Frost the cake again with white chocolate ganache. Add patches of milk chocolate ganache on the sides and smooth lightly with a scraper.

Pour chocolate ganache over the top and spread gently.

Drizzle melted white chocolate and swirl for a marble effect.

Chill for 20 minutes before slicing.