Adapted from mms.com
1 cup butter, at room temperature
1/2 cup sugar
1/2 cup packed brown sugar
2 teaspoons vanilla
1 cups flour
1/2 teaspoon baking soda
1/8 teaspoon salt
1 1/2-2 cups M&Ms
Preheat oven to 350º. Line baking sheets with parchment paper and set aside.
Cream butter and sugars till light and fluffy. Beat in egg and vanilla. Stir in flour, baking soda, and salt till thoroughly combined. Add 1-1/2 cups of M&M's and mix in.
Scoop out balls equal to about 2 tablespoons (I used a cookie disher). Top with 3-5 M&M's. I found that the logo M&M's do not hold up to heat...if you use those, you need to press your reserved candies into the cookies immediately after they're removed from the oven.
Bake about 12 minutes. Cool for a couple minutes on cookie sheet, and then remove to cooling rack. Makes about 2 dozen large cookies.