Preheat your oven to 350°F (175°C). Line a 9x13 inch baking pan with parchment paper, leaving some overhang on the sides for easy removal later.
In a large mixing bowl, cream together the softened unsalted butter, granulated sugar, and packed light brown sugar using an electric mixer. Beat until the mixture is light and fluffy, which should take about 2-3 minutes.
Add the large egg and pure vanilla extract to the butter-sugar mixture. Continue mixing until fully incorporated.
In a separate bowl, whisk together the all-purpose flour, instant vanilla pudding mix, baking soda, and salt. Gradually add this dry mixture to the wet ingredients, mixing until just combined.
Fold in the semi-sweet chocolate chips and red and green M&Ms using a spatula. Be careful not to overmix; you want to maintain the texture.
Spread the cookie dough evenly into the prepared baking pan. Use a spatula to smooth the top, making sure it’s evenly distributed.
Bake in the preheated oven for 20-25 minutes, or until the edges are golden brown and a toothpick inserted into the center comes out clean.
Once baked, remove from the oven and allow the cookie bars to cool in the pan for about 10 minutes. Use the parchment paper overhang to lift the bars out of the pan and onto a cooling rack to cool completely.
