Preheat oven to 350°F.
Cream together butter and sugar until light and fluffy; add egg and vanilla.
Combine flour and baking soda; add to creamed mixture just until blended. Stir in M&M's or chocolate chips.
Shape dough into 1 inch balls. Place on greased cookie sheets about 2 inches apart.
With bottom of a glass dipped in sugar, flatten dough very slightly so as not to crush M&M's.
Bake 10 to 12 minutes or until edges are light golden; let cool 1 minute before transferring to wire rack to cool completely.
Store in tightly covered container.