Preheat and prepare the pans

• Preheat the oven to 180°C (350°F).
• Grease and line three 20 cm cake pans with parchment paper.

Make the chocolate cake batter

• Whisk the flour, granulated sugar, cocoa powder, baking powder, baking soda and salt together in a large bowl.
• Add the eggs, vegetable oil and buttermilk and mix until smooth.
• Pour in the hot water and whisk gently until the batter is thin and glossy.

Bake the cake layers

• Divide the batter evenly between the prepared pans.
• Bake for 30 minutes, until a skewer inserted in the centre comes out clean.
• Leave to cool in the pans for 10 minutes, then turn out onto a cooling rack to cool completely.

Make the American buttercream

• Beat the softened butter in the stand mixer until pale and creamy.
• Gradually add the sifted powdered sugar and beat until fully incorporated.
• Add the milk and vanilla extract and beat for 5 minutes until super smooth and fluffy.

Split the buttercream in two

• Divide the buttercream between two bowls.
• Beat the melted dark chocolate into one half to make the chocolate buttercream.
• Leave the other half plain white, adding a little food coloring if needed.

Fill and stack the cake

• Place the first cake layer on a cake board and spread over a layer of buttercream.
• Repeat with the remaining layers, alternating the buttercream flavours.
• Chill the stacked cake for 30 minutes to firm up.

Crumb coat and finish

• Cover the whole cake with a thin crumb coat of buttercream and chill for 20 minutes.
• Apply the final smooth coat, using the white buttercream on one half and the chocolate buttercream on the other.
• Smooth the surface with the offset spatula on a turntable and clean up the edges.

Decorate and serve

• Pipe a border or add decorations along the seam between the two halves.
• Slice and serve at room temperature.