Preheat oven to 350 degrees. In a large bowl, cream butter and both sugars until well blended. Add the egg and vanilla and mix to combine. In a separate bowl, sift the flour, baking soda and salt together. Slowly add these ingredients into the butter mixture and stir until combined. Fold in M&M candies.
Chill the dough for 1 hour or overnight. Drop dough by rounded tablespoons onto a lightly greased cookie sheet, about 2-inches apart. Bake for 8-10 minutes for chewy or 12-14 minutes for crispy cookies. Makes 24-30 cookies.