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.