Begin by preheating your oven to 350°F (175°C). Line two baking sheets with parchment paper.
In a large mixing bowl, add the softened butter, packed brown sugar, and granulated sugar. Cream these together on medium speed until light and fluffy, about 3-4 minutes.
Add in the large egg and pure vanilla extract. Beat until well combined, ensuring no streaks of egg remain.
In a separate bowl, whisk together the all-purpose flour, baking soda, and fine sea salt. Gradually incorporate this dry mixture into the wet mixture, mixing until just combined.
Gently fold in the mini chocolate chips and Christmas-themed M&M's using a rubber spatula.
Using a cookie scoop or tablespoon, portion out the dough onto the prepared baking sheets, spacing each cookie about 2 inches apart.
If desired, sprinkle colorful sprinkles on top of each cookie before baking.
Bake in the preheated oven for 10-12 minutes, monitoring closely for lightly golden edges and soft centers.
Once baked, let the cookies cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.
Notes
For a delightful holiday display, stack the warm cookies on a festive plate and drizzle with melted chocolate.