Preheat the oven to 375°F and prepare two baking sheets with parchment paper or silicone baking mats. In a large mixing bowl, cream the softened butter with the packed brown sugar and granulated sugar for about a minute using an electric mixer. Incorporate the eggs and vanilla extract, blending until fully integrated. Gradually add the flour, baking soda, and salt, mixing only until the dough forms. Gently fold in the chocolate chips and M&M’s. Chill the dough in the refrigerator for at least 15 minutes, up to 24 hours if time permits. Using a large scoop (approximately 3 tablespoons), portion the dough onto the baking sheets, leaving about 2 inches between each mound. Top with additional M&M’s if desired. Bake for 7–10 minutes, until the edges are lightly golden. Let the cookies rest on the baking sheet for 2 minutes before transferring to a wire rack to cool completely. Store in an airtight container for up to three days.