Step 1: Preheat your oven to 350°F (175°C) and line two large baking sheets with parchment paper or silicone mats.
Step 2: In a large mixing bowl, beat the softened butter, granulated sugar, and brown sugar together until light and fluffy, usually about 3-4 minutes.
Step 3: Add the eggs one at a time, beating well after each addition, then stir in the vanilla extract.
Step 4: In a medium bowl, whisk together the flour, baking soda, and salt.
Step 5: Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Do not overmix.
Step 6: Gently fold in 1 cup of the M&M candies using a spatula.
Step 7: Scoop rounded tablespoons of dough (about 2 tablespoons each) onto the prepared baking sheets, spacing them 2 inches apart.
Step 8: Press the remaining 1/2 cup of M&Ms and the candy eyeballs into the tops of the dough balls to create the monster faces.
Step 9: Bake for 9-11 minutes, or until the edges are lightly golden but the centers still look slightly soft.
Step 10: Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.