1. Line a brownie tin with greaseproof paper, leaving an overhang on each side for easy lifting later.
2. Line with cling film, bottom and sides.
3. Place 1L of softened vanilla ice cream in the bottom of the tin. Smooth out. Freeze until solid and smooth out again so this layer is completely flat.
4. In the meantime, blend together all the ingredients for the crumble and bake on a lined baking sheet for 10-12 minutes at 190°C. Allow to cool completely.
5. Blend together the raspberries and icing sugar and pass through a sieve.
6. Mix the puree with 500ml vanilla ice cream and pour on top of the vanilla layer.
7. Sprinkle over the cooled crumble.
8. Freeze until solid.
9. Cut in to slices and serve.