Preheat your oven to 350 degrees and line 2 baking sheets with parchment paper
Place the butter and sugar in a stand mixer fitted with the whisk or paddle attachment (or simply use a hand mixer). Beat on medium-high speed until light and fluffy (2-3 minutes). Add the eggs and vanilla and beat until blended.
Add the flour, baking soda and salt. Blend on low until the flour is incorporated.
Add in the M&M's and stir with a wooden spoon just until blended (work fast, so the colors don't bleed into the dough).
Using a 2 tablespoon dough scooper (or just a large spoon), scoop onto your prepared baking sheet. Don’t flatten them. Top with a few more M&M's and sprinkle with flaky sea salt or kosher salt if you'd like.
Bake them until lightly browned, 10-12 minutes. Cool on the pan for 3-4 minutes then transfer to a wire rack to cool and enjoy!