Preheat your oven to 350F and spray a 24-count mini muffin tin with baking spray*
Combine butter and chocolate chips in a large, microwave-safe bowl. Microwave for 30 seconds. Stir well, microwave for another 15 seconds and then stir well again. Repeat for 15 seconds until chocolate and butter are completely melted and well-combined.
Stir in cocoa powder and instant coffee (if using).
Add sugars and stir again until combined.
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, baking powder, and salt.
Add dry ingredients to wet and stir until well-combined.
Drop batter by about 1 ½ Tablespoon into prepared mini muffin tin (I like to use a cookie dough scoop to do this, just makes the process cleaner and easier).
Transfer to 350F (175C) oven and bake for 18 minutes or until a toothpick inserted in the center comes out clean or with a few fudgy crumbs (not wet batter!).
If filling with ganache, immediately take a rounded teaspoon or Tablespoon and press into each brownie bite to make a well (if not filling, then simply allow brownie bites to cool and then enjoy!).
Allow brownies to cool for about 10 minutes before removing from muffin tin and placing on cooling rack. Prepare ganache.
Combine 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).