Cake crumb cookies: preheat the oven to 180 C (160 C fan) and line two flat oven trays with baking paper. Put the leftover cake into a food processor and blitz to fine crumbs.
In a large bowl, cream together the butter and sugar. Once light and fluffy, beat in the egg, vanilla, salt and baking soda, then fold in the cake crumbs, flour and chocolate chips. Add a few tablespoons of milk, if needed, to bring the dough together.
Scoop 2 tbsp dough at a time and place, well spaced, on your lined baking trays. Bake for 10-12 minutes until spread out and golden brown. Leave to cool on the trays for a few minutes then move on to a wire rack to cool completely.