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Break off chunks of dough (about the size of a cherry tomato) and arrange them, spaced apart, on the sheets. Press the cookies down with the back of a fork to squash them a little. (The cookies can be frozen for up to two months – to cook from frozen, add an extra 1-2 mins to the cooking time.) Bake for about 12 mins until golden around the edges and paler in the centre. Leave to cool on the trays for 5 mins.
Meanwhile, put the bananas, cream and vanilla in a food processor and blend until they make a thick ice cream. Scoop into balls with an ice cream scoop, and sandwich between the cookies. Serve immediately.