Preheat your oven to 350°F (175°C).

In a large bowl, cream together the softened butter, granulated sugar, and brown sugar until smooth.

Beat in the eggs, one at a time, then stir in the vanilla.

In another bowl, whisk together the flour, baking soda, and salt. Gradually blend this dry mixture into the wet mixture.

Fold in the M&M's and chocolate chips if using.

Drop tablespoonfuls of dough onto ungreased baking sheets.

If desired, sprinkle some additional M&M's or sprinkles on top of each cookie.

Bake for 10-12 minutes or until the edges are golden.

Let them cool on the sheets for a few minutes before transferring them to a wire rack to cool completely.