Combine the eggs, both sugars, vanilla, corn syrup, baking soda, and salt in a large mixing bowl. Stir in the melted butter.
Next add the peanut butter, oats, and flour and mix thoroughly. Stir in the chocolate chips, butterscotch chips, and M&Ms. The dough will be a bit loose.
Chill the dough for about 30 minutes to allow the oats to absorb the butter.
Preheat oven to 350°F (175°C). Line baking sheets with silicone liners or parchment paper.
Drop the dough by 4-tablespoon portions onto the prepared baking sheets. Flatten the top of each cookie slightly.
Bake (one pan at a time) for 12 to 15 minutes or until lightly browned. Keep the remaining dough refrigerated between batches.
Cool the cookies on the pan on a wire rack for about 10 minutes, then carefully transfer the cookies from the pan directly to a wire rack to cool completely.
Video
Notes
Yield: Makes around 24 large cookies.
To store: Store these monster cookies in an airtight container on the counter for 2 to 3 days.
To freeze: You can freeze monster cookies in an airtight, freezer-safe container or freezer bag for up to 3 months. Let the cookies thaw at room temperature or warm them up in the microwave.
Freezing the cookie dough: The dough can also be portioned and frozen for baking later; just add a few extra minutes to the baking time.