Preheat the Oven:

1. Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper or use a nonstick baking sheet.

Cream the Wet Ingredients:

1. In a large mixing bowl, combine the softened butter, brown sugar, granulated sugar, egg, and vanilla extract.
2. Use an electric mixer to cream the ingredients together until smooth and fluffy.

Mix the Dry Ingredients:

1. Add the all-purpose flour, salt, and baking soda to the wet mixture.
2. Continue mixing with the electric mixer until a sticky dough forms.

Add the M&Ms:

1. Stir in the M&Ms and mini M&Ms until evenly distributed throughout the dough.

Form the Cookies:

1. Roll the cookie dough into balls approximately 1 1/2 inches in diameter.
2. Place the dough balls on the prepared baking sheet, leaving at least 2 inches of space between each ball.

Bake the Cookies:

1. Bake in the preheated oven for 10-12 minutes, or until the cookies are golden brown around the edges.

Cool and Serve:

1. Remove the cookies from the oven and let them cool on the baking sheet for a few minutes.
2. Use a spatula to transfer the cookies to parchment paper to cool completely.
3. Serve and enjoy!