Preheat oven to 180°C. Line 2 x 8"/20cm cake tins with baking parchment on the base.
In a large bowl combine the dry ingredients - flour, cocoa, baking powder, bicarb, salt and caster sugar. Mix to evenly disperse.
In another bowl combine the wet ingredients - milk, veg oil, brown sugar, vanilla extract and eggs. Whisk until smooth and no lumps of brown sugar remain.
Pour the wet ingredients into the dry and mix until a smooth batter. Split between the two prepared baking tins. Give the tins a little shimmy to even out the batter. Bake for 25 - 30 minutes. Check if cooked, by piercing with a skewer. Skewer should come out clean.
Once removed from the oven let them cool for 5 minutes in their tins before turning out onto wire racks to cool completely.
To prepare the frosting, melt the chocolate in a bowl suspended over a pot of boiling water or by microwaving in 15 second blasts and giving a stir between each. Once melted set aside to cool.
Meanwhile, Add the butter to a bowl and mix with an electric mixer until pale and light. Mix in the vanilla extract, then sift in icing sugar and mix on a low speed to prevent it flying everywhere. Add in the slightly cooled chocolate and milk and continue to beat until a smooth spreadable texture is achieved. (you may need to add an additional tbsp of milk)
When cake have cooled completely, Lay one layer, dome side down, on a plate or cake stand. Slather with 2/3 of the frosting. Pushing right to the edges. Sandwich together with the other layer dome side down so that you have a flat surface on top and spread the remains of the frosting on top and sprinkle around the edges with sprinkles if desired.