Preheat the oven to 350℉. Line 2 baking trays with parchment paper and set aside.
In a medium sized bowl, whisk together the melted and slightly cooled butter and brown sugar. Add in egg and vanilla and continue whisking.
1/2 cup butter, 1/2 cup brown sugar, 1 egg, 1 tbsp vanilla
Add the flour, oats, baking soda, and salt into the bowl. Using a spatula, gently fold in the ingredients until a soft dough forms.
3/4 cups all-purpose flour, 3/4 cups rolled oats, 1/2 tsp baking soda, 1/4 tsp salt
Add in the Christmas M&M's and mix a few more times until incorporated.
1/2 cup Christmas M&Ms
Roll the dough into balls and place 2 inches apart on the prepared baking tray. Gently flatten the dough down. Bake at 350°F for 10 minutes. The cookies will look set around the edges and soft inside.
Let the cookies cool on the baking tray. They will continue to bake as it sits on the warm baking tray. Sprinkle with flaky salt (if desired). Eat warm or let cool and store in an airtight container for up to 5 days.