Preheat your oven to 350°F (176°C) and spray all cups of a standard 24-cup mini muffin tin with non-stick cooking spray.

In a large mixing bowl, cream together softened unsalted butter, brown sugar, and granulated sugar using an electric mixer on medium speed for 3-4 minutes until light and fluffy.

Add the egg and vanilla extract to the mixture and beat on medium speed until fully combined.

In a separate bowl, whisk together the flour, baking soda, cornstarch, and salt, then gradually add to the butter mixture until just combined.

Fold in the chocolate chips, chopped M&Ms, and sprinkles using a spatula.

Scoop about 2 tablespoons of the dough into each cup and press it down into the cup to form a cookie bowl.

Bake the cookie cups for 10-12 minutes, until the edges are lightly golden brown while the centers are still soft.

After 5 minutes of cooling, use the bottom of a tablespoon to press down the center of each cookie cup to deepen the well.

In a clean mixing bowl, beat together the salted butter and shortening until creamy.

Gradually add the powdered sugar and beat until incorporated; then add vanilla and milk/water as needed until a fluffy consistency is reached.

Divide the frosting into two bowls, leaving one white and coloring the other red with gel icing color.

Pipe the frosting into the cooled cookie cups, topping with additional sprinkles and M&Ms as desired.