Start by melting the unsalted butter in your mug. You can do this in the microwave for about 20-30 seconds until it's completely melted.
Add the brown sugar and granulated sugar to the melted butter. Use a spoon to mix the ingredients until well combined and smooth.
Stir in the pure vanilla extract and your choice of either the egg yolk, milk, or egg beaters. Mix until everything is well incorporated.
Sprinkle the baking powder, cocoa powder, and all-purpose flour into the wet mixture. Stir until there are no dry spots remaining.
Gently fold in the mini chocolate chips, ensuring they are evenly distributed throughout the cookie dough.
Place the mug in the microwave and cook for about 40-60 seconds. Start checking at 40 seconds; the cookie should look set but slightly soft in the center. Be careful not to overcook!
Allow your cookie to cool for a minute or so before diving in. This will help enhance the texture and flavor.
