Preheat oven to 180°C. Line 2 baking trays with baking paper.
Use an electric mixer to beat butter and combined sugar in a bowl until pale and creamy. Beat in egg and egg yolk until well combined. Add flour and baking powder and stir to combine. Stir in M&M’s and chocolate.
Roll 1/4 cup portions of dough into balls. Place on the lined trays and gently flatten. Bake, in batches, swapping trays halfway through cooking, for 12-15 mins or until light golden. Set aside on the trays to cool.