Line 2 large baking trays with greaseproof. In a mixing bowl, beat together the butter and sugar until creamy and well blended. Beat in the egg yolk and Vanilla Extract.
Sift the flour and Baking Powder on top and add the M&Ms. Mix together to form a firmish dough. Divide the dough into 12 pieces and form each into a ball.
Place the balls of dough spaced slightly apart onto your prepared baking trays and press them down slightly. Chill the dough in the fridge for 30 minutes.
10 minutes before baking, pre-heat the oven to 180°C/160°C Fan/Gas Mark 4. Bake the cookies for 10-15 minutes until lightly golden. Cool on the trays for 5 minutes before transferring to a wire rack to cool completely, the cookies will firm up as they cool.
Once cooled your cookies are ready to serve and Enjoy!