In a large bowl, cream together butter and sugar. Add egg and vanilla extract. Beat until combined. Add flour, baking soda and salt. Beat again until combined. Fold in 1 cup M & Ms. Using your hands bring the dough together. Cover and refrigerate the dough for 15-30 minutes.

Preheat oven to 190 C or 375 F. Line two baking trays with parchment paper. Scoop small balls of dough (about 3 tbsp) onto the baking sheets. Place them 2” apart to allow enough space for the dough to spread. Place more M & Ms on top of the dough balls. Bake for 8-10 minutes. Allow cookies to set on the pan for 2 minutes. Transfer to a wire rack to cool completely.

Store cookies in an airtight container at room temperature for about 3 days.