Preheat the oven to 350 degrees Fahrenheit. Line a 9x13 pan with parchment paper.
Cream together the butter and sugars in a large bowl for about 3 minutes. 1 cup unsalted butter, 1 cup brown sugar, 1/2 cup granulated sugar
Add in the eggs and vanilla and stir into the creamed butter. 2 large eggs, 2 tsp vanilla extract
Add the dry ingredients to the wet ingredients and stir everything together till combined, and you have a nice soft cookie dough. 2 3/4 cups all purpose flour, 1 tsp baking soda, 1 tsp salt
Add in 1 1/4 cup of the m&ms, and most of the chocolate chips and stir them until mixed evenly into the batter. 1 1/2 cup red and green m&ms, 1/2 cup chocolate chips
Scoop the cookie dough into the prepared pan and press the dough evenly into the corners and all around the pan.
Press the extra M&Ms and chocolate chips onto the top of the cookie dough.
Bake the cookie bars for 25 to 30 minutes, or until the tops are golden, and the middle is set. (You can test the scenters with a toothpick, you want it to come out clean, or with a few crumbs, but no runny dough).
Remove from the oven and let the bars cool in the pan for at least 1 hour, or completely. Then lift the parchment out of the pan, and cut into squares and serve.