Preheat oven to 350°F (175°C) and line baking sheets with parchment paper.
Cream softened butter and sugars in a mixer for 3 minutes until light and fluffy.
Add eggs one at a time, mixing well, then stir in vanilla extract.
Whisk together flour, baking soda, and salt in a separate bowl.
Gradually add dry ingredients to wet ingredients on low speed until just combined.
Fold in 1 cup of M&Ms manually with a spatula.
Scoop 30-gram balls of dough and place them 2 inches apart on sheets.
Press remaining M&Ms into the top of each dough ball.
Bake for 9-11 minutes until edges are set but centers are still soft.
Cool on the pan for 5 minutes before moving to a wire rack.