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Heat oven to 180C/160C fan/gas 4. Tip the butter and sugar into a large mixing bowl, beat until combined, then stir in the yolks and vanilla. Tip in the flour, baking powder, chocolate chips, a pinch of sea salt and any other fillings you want to add. Mix until a crumbly dough forms.
Lightly butter a 25cm ovenproof frying pan. Spoon in and flatten the cookie mixture. For a gooey dessert, bake for 20 mins, leave to rest for 5 mins, then scoop straight from the pan and serve with ice cream, if you like. For a firmer cookie you can cut, bake for 30 mins, then leave to cool completely before cutting into wedges.