Preheat oven and prepare three 9-inch cake tins with parchment paper.
Sift flour, sugar, cocoa, baking soda, baking powder, and salt into a bowl.
Add stout and espresso powder to the dry ingredients.
Cream butter and sugar until light and fluffy.
Add eggs one at a time, beating well after each addition.
Mix in mayonnaise.
Alternate adding dry ingredients and hot water to the batter, mixing well after each addition.
Fold in dark chocolate chunks.
Divide batter evenly between the prepared cake tins and bake for 30-35 minutes.
Let cakes cool completely before decorating.
To make the buttercream, beat butter until light and fluffy.
Gradually add powdered sugar and milk, mixing until smooth.
Place one cake layer on a cake stand and spread a generous amount of buttercream on top.
Repeat with remaining layers.
Frost the sides of the cake with a thin layer of buttercream.
Decorate as desired with shaved chocolate, fruit, or other toppings.
Chill cake for at least 30 minutes before serving.
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