Preheat the oven to 180oc and grease and line a 20cm cake tin.
Beat together the butter and brown sugar then stir in the oats, flour and baking powder until you have a crumbly mixture.
Press half of the mixture into the base of the baking tin until you have a smooth flat base.
Prick the base all over with a fork then bake for 15 minutes until golden.
Mix the mashed raspberries with the caster sugar and cornflour and spread over the cooked base.
Sprinkle the remaining crumble over the top then return to the oven and cook for a further 30 minutes.
Cool for ten minutes then slice whilst in the tin. Leave to cool fully in the tin then remove and serve.