Using an electric mixer cream together the butter, granulated sugar, brown sugar and vanilla. Best on medium high until light beige in color around 2-3 minutes.
Add the eggs one at a time beating well after each addition. Scrape the sides of the bowl periodically, as needed.
In a separate bowl sift together the flour, baking soda and salt. Add to the creamed mixture gradually. Beat until fully incorporated. Batter will be stiff.
By hand mix in the white chocolate chips and M&M's until evenly distributed. Cover and chill for 2 hours or overnight.
Preheat oven 375°F. Line two x-large baking sheets with parchment. Set aside.
Use a medium ice cream scoop or Tbsp to divide dough. Leaving at least 2 inches between cookies.
Bake for 10-12 minutes or until lightly golden. Cool on the pan for 5 minutes then to a cooling rack to cool completely. Store tightly sealed at room temperature.