Preheat your oven to 350 degrees Fahrenheit (175 degrees Celsius). Line your baking sheets with parchment paper or baking mats, and set aside.
Place butter in a microwave-safe bowl and heat for 30-45 seconds or until it's almost completely melted but has a few soft pieces left.
In a large mixing bowl, whisk together flour mixture, baking soda, cornstarch, and salt.
In the bowl of a stand mixer (or a large mixing bowl with a hand mixer), beat together butter, sugars, and vanilla for 1 minute.
Then, add the eggs and mix until combined.
Next, add in the dry ingredients. Beat until just combined or until a soft dough forms.
Fold in the chocolate chips, M&Ms, and sprinkles until evenly dispersed.
To bake the cookies, use a standard-size cookie scoop (approximately 1.5 tbsp.) to form balls of dough. Place onto the prepared sheets, leaving at least 2 inches in between. Top with all the additional mix ins if desired.
Place in the oven and bake for 11-13 minutes, rotating the sheet halfway through the baking process. Remove from oven when cookies are golden brown in color around the edges.
Finally serve and enjoy!
Notes
To STORE these Halloween cookies, place them in an airtight container at room temperature for up to one week. FREEZE the dough or baked cookies for up to three months.To REHEAT, place cookies in a preheated oven at 300°F (150°C) for 5-7 minutes or microwave for 10-15 seconds. For frozen dough, bake directly from the freezer, adding an extra minute to the baking time.