1. Preheat the oven to 180°C (160°C Fan, Gas Mark 4) Grease and line a 20cm (8 inch) square cake tin.
2. In a mixing bowl, beat together the butter and brown sugar then stir in the oats, flour and Baking Powder to form a crumbly mixture. Press half of the mixture into the base of the prepared tin to form a smooth base. Prick all over with a fork and bake for 15 minutes until lightly golden.
3. Mix the mashed raspberries with the caster sugar and cornflour and spread over the cooked base. Sprinkle with the remaining crumble topping.
4. Bake for about 30 minutes until lightly golden. Cool for 10 minutes then cut into 25 small squares and allow to cool in the tin before carefully removing to serve.