Cream the softened butter, brown sugar and granulated sugar in a mixer.
Add the eggs and vanilla and mix until light and fluffy.
In a separate bowl, add flour, the Baking Soda and salt and mix.
Add the flour mixture to the cookie dough a bit at a time and mix until it is completely incorporated. (Do not overmix.)
Stir in the M&M Minis by hand.
Bake the cookies in a 350 degree oven for 10-12 minutes.
