Preheat oven to 350 degrees. Line a baking sheet (half or quarter sheet) with parchment paper or a silicone baking mat.
Place butter in a microwave safe mixing bowl.
Heat in microwave until about 2/3 of the way melted (it should take roughly 20 seconds), remove and stir to melt fully.
To melted butter, add brown sugar, salt, 1 tbsp egg, vanilla, flour and baking soda. Stir with a spoon or rubber spatula until well combined.
Mix in about 3/4 of the M&Ms. Shape dough into a disk about 3 1/2-inches on a prepared baking sheet.
Sprinkle remaining 1/4 of the M&Ms over top along with sprinkles if using. Bake in preheated oven 11 - 14 minutes, until edges are just golden brown (cookie should appear slightly under-baked in center).
*To get 1 Tbsp egg crack egg into a small bowl, whisk until yolk and white are blended, then measure out 1 Tbsp. Reserve remainder for another use.
**I've also made this using a 1.77 oz tube of red and green Mini M&Ms, which is equal to 1/4 cup.
Recipe source: Cooking Classy - inspired by my other single cookie