Cream butter and sugars: In a large bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy,1 about 2-3 minutes using an electric mixer.
Add eggs and vanilla: Beat in the eggs one at a time, then stir in the vanilla extract.
Combine dry ingredients: In a separate bowl, whisk together the flour, baking soda, and salt.
Combine wet and dry ingredients: Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Do not overmix.
Add chocolate: Stir in the chocolate chips, M&Ms, and dark chocolate chunks (if using).
Chill the dough: Cover the dough and refrigerate for at least 2 hours, or preferably overnight. This chilling period is crucial for preventing spreading and enhancing the flavor.
Preheat and prepare: Preheat oven to 375°F (190°C). Line baking sheets with parchment paper.
Scoop and bake: Use a large cookie scoop (about ¼ cup) to scoop the dough onto the prepared baking sheets, leaving at least 2 inches between cookies.
Bake: Bake for 12-15 minutes, or until the edges are golden brown and the centers are still slightly soft. Be careful not to overbake.
Cool: Let the cookies cool on the baking sheets for a few minutes before transferring them to a wire rack to cool completely.