Step 1: Prepare the baking sheet
Line a small baking sheet with parchment paper and set aside.

Step 2: Cream butter and sugars
In a large bowl, beat the softened butter with brown sugar and granulated sugar until light and creamy.

Step 3: Add egg and vanilla
Beat in the egg and vanilla extract until the mixture is smooth and thoroughly incorporated.

Step 4: Mix dry ingredients
In a separate bowl, whisk together flour, cornstarch, baking soda, and salt.

Step 5: Combine wet and dry
Gradually add the dry ingredients to the wet mixture, stirring just until combined. Do not overmix.

Step 6: Add chocolate and candies
Fold in the semi sweet chocolate chips and M and M candies evenly throughout the dough.

Step 7: Scoop and shape
Scoop about 2 tablespoons of dough and roll into balls. Place on the prepared baking sheet. Press a few extra candies on top for a bakery style finish.

Step 8: Freeze
Cover and freeze the dough balls for at least 1 hour or up to overnight. If freezing overnight, allow the dough to sit at room temperature for 10 minutes before baking.

Step 9: Bake
Preheat oven to 350°F. Place chilled dough balls on a parchment lined baking sheet and bake for 10 to 12 minutes. The centers should look slightly soft when removed.

Step 10: Cool
Allow cookies to cool on the baking sheet for several minutes before transferring to a wire rack.