Preheat your oven to 350°F (175°C) and prepare a baking sheet by lining it with parchment paper.

In a large mixing bowl, cream together the softened butter, dark brown sugar, granulated sugar, and vanilla extract until the mixture is light and fluffy. Gradually beat in the eggs and egg yolk, adding them one at a time, and mixing thoroughly after each addition.

Sift in the all-purpose flour, baking soda, baking powder, and salt. Mix until just combined, then fold in the oats to create a cohesive dough.

Gently incorporate 1 cup of the M&M's, crushed pretzels, semi-sweet chocolate chips, and white chocolate chips. Stir until evenly distributed throughout the dough.

Shape the dough into 1/4-cup-sized balls and arrange them on the prepared baking sheet, spacing them about 3 inches apart to allow room for spreading.

Place the baking sheet in the preheated oven and bake for 8 minutes. Rotate the pan to ensure even cooking and bake for an additional 2 minutes. Remove the pan and carefully press the remaining M&M's onto the tops of the cookies for a festive touch. Return to the oven and bake for another 2–3 minutes until the edges are set but the centers remain slightly soft.

Take the cookies out of the oven and allow them to cool on the baking sheet, where they will continue to firm up. Enjoy warm for a gooey, decadent treat, or let them cool completely before storing in an airtight container for up to four days.