Line an oblong traybake tin (21 x 30cm) with baking parchment and heat oven to 180C/fan 160C/gas 4. Beat the butter, sugar, flour, baking powder, eggs and milk together until creamy. Spoon into the tin, level the top, then bake for 25 mins until almost set.
Meanwhile, get the yogurt and fruit at the ready and make the crumble by rubbing all the ingredients together.
When cake has baked for 25 mins, whip it out of the oven. Working quickly, spoon over the yogurt, scatter over the fruit, then top with the crumble. Return to oven for 15-20 mins until a skewer comes out clean. Serve warm as a pudding, or cooled and cut into squares with tea.