In a large bowl, cream butter and both sugars until well blended. Add the egg and vanilla extract and mix to combine.
In a separate bowl, sift the four, baking soda and salt together. Slowly add these ingredients into the butter mixture and stir until combined.
Fold in the M&M's and chill the dough for 1 hour or overnight.
*Drop dough by rounded tablespoons onto a lightly greased tray about 2 inches apart.
Bake for 8 to 10 minutes for chewy cookies or 12 to 14 for crispy cookies.
*For a large cookie, spray a 12-inch pizza pan with non-stick spray. Turn dough onto the pan, using floured hands, pressed the dough to the edges until dough is even and as smooth as possible.
Bake 27 minutes or until edges are browned and the center is set.