Heat the oven to 180C/160C fan.
Grease and line the base of a 20 cm cake tins with baking parchment.
Beat the butter and sugar together until fluffy. Add the eggs gradually, beating well after each addition. Fold in the flour, baking powder, mint, lime zest and cocoa powder. Add the milk, lime juice and vanilla and combine.
Spoon the batter into the tin and bake for about 30-35 minutes or until a skewer inserted into the centre comes out clean.
Let the cakes rest in tin for 5 minutes the turn out onto a wire rack and peel off the parchment paper. Let it cool completely.
While the cake is in the oven, make the chocolate icing.
Melt the chocolate over simmering water. Take it off the heat, then add the butter, cocoa powder, vanilla. Add some milk until you get a thick chocolate sauce.
Let it cool.
When the cake has cooled, spread the icing over the cake, then dust it with some lime zest and garnish with mint leaves.