Mix butter and sugar until creamy. Beat in egg. Add the rest of the dry ingredients and mix. It will seem crumbly at first, but keep mixing--it will come together to be a thick dough. Smash the M&M candies with a rolling pin (put them inside a large plastic bag), then pour them into the dough and combine well. Make sure to get all the bits, pieces, and shavings--that's what makes them so good!!! Bake 10-12 minutes at 350 degrees. Cool on a wire rack