Preheat the oven to 350℉/176℃ and grease a muffin tin well with oil (or use muffin liners) and set aside.
In a bowl, add in flour, baking soda, baking powder, cornstarch, cinnamon, and salt. Stir to combine and set aside.
1 3/4 cups all-purpose flour, 1/2 tsp baking soda, 1/2 tsp baking powder, 1/2 tsp cornstarch, 1/4 tsp cinnamon, 1/4 tsp salt
In a bowl, add the melted butter, brown sugar, and white sugar. Whisk together for 1-2 minutes until well combined.
3/4 cups unsalted butter, 1/3 cup dark brown sugar, 1/3 cup white sugar
Add in the egg, egg yolk, and vanilla. Whisk together for another minute or so until smooth.
1 large egg, 1 large egg yolk, 1 tbsp vanilla
Mix the dry and wet ingredients together until a soft cookie dough forms. Do not over-mix!
Add in your favourite chopped up chocolate candies and mix in a few times until evenly distributed.
1 1/2 cups chocolate candy
Scoop evenly between the muffin tin, and add a few more chopped up chocolates on top if desired.
Bake for 18-22 minutes or until a toothpick inserted in the middle comes out clean. Allow to cool, then gently run a butterknife around the edges of each cookie cup to 'pop' them out. Enjoy!