Preheat oven to 350 degrees. Lightly coat baking sheets with non-stick cooking spray.

In a medium bowl, whisk together flour, salt, baking soda and baking powder; set aside.

With an electric mixer, beat butter, peanut butter and sugars in a large bowl until smooth. Add egg and vanilla and mix well.

Add flour mixture to peanut butter mixture and stir to combine. Mix in M&M's. Cover dough and refrigerate for about 30 minutes.

Using a large scoop (such as an ice cream scoop, or 1/4 cup measuring cup), drop dough onto prepared cookie sheets, 2 inches apart. Flatten to about 3/4 inch thick.

Bake for 10-13 minutes, or until golden and just set. Cool on pans 5 minutes before removing to wire racks to cool completely.