Preheat oven to 350F (175C) and prepare two 24-cavity mini muffin tins by spraying with baking spray or lightly greasing and flouring.
Combine melted (cooled) butter and sugars in a large bowl, stir well.
Add eggs, and egg yolk one at a time, stirring well after each addition.
Stir in vanilla extract.
In a separate, medium-sized bowl, whisk together flour, cornstarch, baking powder, and salt.
Gradually stir dry ingredients into wet until completely combined. Stir in chocolate chips.
Use a 1 1/2 Tbsp-sized cookie scoop to drop batter into prepared muffin tin wells.
Bake on 350F (175C) for 18 minutes or edges are just beginning to turn golden brown.
Allow cookie cups to cool for 5 minutes, and then use a rounded teaspoon to press into the center of each brownie bite and create a well (press a little more than halfway into each cookie).
Allow brownie bites to cool another 10-15 minutes in the muffin tins, and then gently remove to cooling rack to cool completely.
Prepare your ganache.
GanacheCombine chocolate chips, heavy cream, and butter in a small saucepan over medium-heat.
Stir frequently until chocolate is completely melted and mixture is smooth.
Remove from heat and stir in vanilla extract, bourbon (if using), and salt. Stir well.
Allow mixture to cool slightly at room temperature, about 10 minutes.
Use a small spoon to pour ganache into the indent of each brownie.
Allow ganache to firm up before eating (about 1-2 hours -- or you can enjoy them warm, it'll just be a bit messier).