Sift together the cake flour and baking powder, add the brown sugar and lightly stir in the berries. Fold in the yoghurt mixture. Lightly brush a muffin tray with butter and line with wax paper squares.
Spoon the muffin mixture into the lined muffin tray until each mould is ¾ full. Top each one with berries and sprinkle with ½t caster sugar. Bake at 150°C for about 25 minutes.
Serve with double-thick cream or thick sour cream.