In a large mixing bowl, beat the butter and sugars together with an electric or stand mixer until light and fluffy. Scraping down the sides of the bowl with a silicone spatula. Beat in the eggs and vanilla until creamy.
Turn the speed of the mixer to low and add flour mixture just until blended. Gently stir in 1 cups of M&Ms by hand with a spoon *see notes*.
Drop dough by rounded tablespoons (or with a small scoop) onto prepared cookie sheets. Bake cookies in the middle rack of the oven for 10 to 12 minutes or until the cookies are golden brown around the edges, and just set in the center.
Remove from the oven and let rest for 5 minutes before transferring the cookies to a wire rack. Gently press in remaining cup of M&Ms into cookies before cooling *see notes*.
The M&Ms will crack and melt slightly during baking. Adding half of the M&Ms after baking creates cookies with intact M&Ms and make a better presentation.These cookies will keep well for up to 3 days at room temperature if stored in an airtight container. Although, I've never known them to last that long around our house.
M and M cookies are great to freeze, too! To keep the freshest I recommend you first wrap the cookies individually with plastic wrap, and then store together in an airtight freezer bag for up to 3 months.