Grease and line three 8-inch (20 cm) cake tins with parchment paper.
In a large mixing bowl, add the flour, cocoa powder, baking powder, bicarbonate of soda, caster sugar, and brown sugar. Stir until all the dry ingredients are evenly mixed.
Melt the chocolate and butter together in a heatproof bowl set over a saucepan of simmering water, or microwave in short bursts until smooth.
225 Grams Dark chocolate, 225 Grams Unsalted Butter
Dissolve the coffee granules in 125 ml boiling water. Stir the coffee into the melted chocolate mixture until combined.
1 Teaspoon Instant coffee granules
In a separate bowl, whisk together the eggs and buttermilk.
4 Medium Eggs, 75 milliliters Buttermilk
Add both the chocolate mixture and the egg mixture to the dry ingredients. Mix until the batter is smooth, runny, and lump-free.
Divide the batter evenly between the three cake tins.
Bake in the preheated oven for 25–30 minutes. Check doneness by inserting a skewer into the center; if it comes out clean, the cakes are ready. If the skewer has batter on it, bake for a few more minutes and test again.
Allow the cakes to cool in their tins for 5 minutes. Transfer the cakes to a wire rack to cool completely.
To Make The Buttercream
To make the buttercream, beat the butter for 2 minutes using a mixer until creamy.
340 Grams Unsalted butter
Add the powdered sugar gradually on low speed until fully combined.
720 Grams Powdered sugar
Mix in the salt, vanilla, and milk or cream. Increase to medium speed and beat for about 3 minutes until light and fluffy. For softer frosting, add a little more milk, half a tablespoon at a time.
Set aside a small portion of the buttercream and mix with green food coloring.
Green food coloring
Tint the remaining buttercream pink.
Pink food coloring
To Assemble the Cake
Place the first cake layer on a serving plate or cake stand. Spread about 1 cup of pink frosting on top.
Add the second cake layer and repeat with another cup of frosting.
Place the third cake layer on top. Frost the top and sides of the cake with the remaining pink frosting.
Use a toothpick to trace the words “HAPPEE BIRTHDAE HARRY” on top.
Pipe the letters over with the green frosting. For Hagrid’s rustic look, drag a toothpick across the words to create “cracks.”
Serve and enjoy the magic!
Notes
Tips and Tricks
Room temperature ingredients: Make sure your butter, eggs, and buttermilk are at room temperature for a smoother batter and fluffier cake.
Don’t skip the coffee: The coffee won’t make the cake taste like coffee, it simply enhances the chocolate flavor and makes it richer.
Level your cakes: If your cake layers bake with domes, gently trim the tops with a serrated knife before stacking for a more stable cake.
Frosting consistency: If your buttercream feels too stiff, add an extra splash of milk. Too runny? Add a little more powdered sugar until it’s pipeable.
Rustic charm: Don’t worry about making the frosting perfectly smooth, the original cake Hagrid made had a homemade, slightly messy look, which adds to its charm.