Heat oven to 180C/160C fan/gas 4.
Grease a 20 x 30cm tin, then line with
baking parchment. Beat the butter, flour,
sugar, baking powder, eggs, yogurt, milk
and vanilla until lump-free. Stir in the
chopped chocolate. Spoon into tin; bake
for 25-30 mins until golden and risen and
a skewer comes out clean. Cool in the tin.
Melt the rest of the chocolate. Stir icing
sugar into soft cheese, then the chocolate
until smooth. Chill, spread over the cake,
then cut into squares and decorate.