Preheat the oven to 200C/Fan 180C/Gas Mark 6. Lightly oil then line the base and sides of a loose based 20cm/8in cake tin. Mix the crushed biscuits with the butter and tip into the base of the tin. Smooth with the back of a spoon. Chill for 10 mins.
Mix together the sugar, cornflour and vanilla seeds. Tip into a food processor with the remaining ingredients and pulse until smooth. Alternatively beat them all together in a large bowl with a wooden spoon. Pour into the chilled cake tin then bake for about 45mins
Cool in the tin. when cold, stand the tin on a glass so that the base of the tin is pushed up and remove the sides. Peel away the lining paper, transfer to a plate and chill until required. To serve, cut into wedges and serve with raspberry coulis.