Preheat the oven to 180°C/160°C (fan)/gas mark 4. Grease and line a 15cm round cake tin.
Whisk together the butter, caster sugar, and brown sugar until light and fluffy.
Whisk in the eggs one at a time and add the vanilla extract.
Fold in the cream and flour gradually, alternating between the two (a splash of cream, then a spoonful of flour, and repeat).
Fold in the black cocoa powder, milk, and salt. Transfer the batter to the prepared cake tin.
Bake for 45-60 minutes, or until a skewer inserted into the center comes out clean.
Leave the cake to cool in the tin for 10 minutes, then transfer it to a wire rack to cool completely.
For the frosting, beat together the butter, icing sugar, cream cheese, and vanilla extract for 2-3 minutes until smooth. If not thick enough, add more icing sugar as needed.
Once the cake is completely cool, divide it into layers and fill with the frosting. Decorate with sprinkles if desired and serve.