Preheat oven to 350 degrees. Line baking sheets with parchment or nonstick baking mat; set aside.
Combine flour, baking soda and salt in a small bowl; set aside. In a large mixing bowl, beat butter, granulated sugar, brown sugar and vanilla extract until creamy. Add eggs, one at a time, beating well after each addition. Gradually beat in flour mixture. Stir in 1 cup of the white chocolate chips.
Roll about a tablespoon of batter and drop onto ungreased baking sheets. (Cookies will spread, so make sure you have enough space.) Gently press down the batter and add in 3-4 M&M's and 4-5 white chocolate chips to the top of the cookie.
Bake for 9 to 11 minutes or until golden brown. Cool on baking sheets for 4 minutes and remove to wire racks to cool completely.
To turn into ice-cream sandwiches, simply scoop vanilla ice-cream onto a cookie and add another cookie on top. Sprinkle the sides with festive sprinkles, if desired. Freeze up to a day in advance.