Grease and line with parchment paper a 12-15cm diameter round cake tin. Preheat the oven to 160˚C.
In a bowl, beat the egg with the caster sugar until light and fluffy.
Stir in the oil, milk, and cold coffee. Sieve and Fold in the flour, cocoa powder, baking powder. Stir until combined into a stiff mixture.
Pour the mixture in the tin and place in the oven.
Bake for 35-45 minutes until a skewer inserted in the middle comes out neatly. Remove from the oven and set aside to cool.
Prepare the buttercream.
Melt the chocolate in a bowl, over a bain-marie ( saucepan with simmering water). Once, melted, stir in the warm water. And keep aside.
In a bowl, beat the butter until light and fluffy, then fold in the melted chocolate mixture next, beat in the icing sugar until fully combined light and fluffy.
Slice the cake sideway to have 2 layers of cake and spread half of the buttercream equally over both layers.
Place one layer on top of the other to create the cake and spread the rest of the buttercream around the sides and top to cover the cake with the buttercream icing.
Sprinkle chocolate curls around and set aside for 2 hours to settle the flavours.
Serve the cake within 2-3 days for best flavour and texture.
