In a large bowl cream butter and sugars until light and fluffy; beat in egg and vanilla.
In a medium bowl, combine flour, baking soda and salt and blend into creamed mixture a little bit at a time and mix well. Stir in M & M. Drop by heaping tablespoons about 2 inches apart onto non-stick cookie sheet. Bake 10 to 13 minutes or until edges are lightly browned and centers are still soft. Do Not Over Bake. Cool one minute on cookie sheets; cool completely on wire racks. Store in tightly covered container. Makes about 3 dozen 3-inch cookies.