Preheat oven to 180°C and line a 25cm round cake tin with baking paper and spray with non-stick cooking spray.
Add butter, sugar, eggs, milk, flour, vanilla, cocoa powder and baking powder to a bowl and beat until smooth and well combined.
Pour batter into prepared cake tin and bake for 45-50 minutes or until a skewer comes out clean. Place on a cooling rack and allow to cool completely.
Meanwhile, roughly chop chocolate and place in a large heat-proof bowl over a saucepan of simmering water. Add cream and stir often until melted and well combined.
Remove from heat and allow to cool slightly before whisking in the icing sugar. Set aside to cool completely. If icing is too runny, place in the fridge to firm up for 5-10 minutes.
Evenly cover the cake with a layer of icing. Store leftover cake in an airtight container for up to 5 days.