Preheat oven to 350℉ and grease a mini muffin tin with cooking spray.
Place the butter in a medium-sized microwave-safe bowl.
1/2 cup butter
Microwave the butter until completely melted.
While the butter is hot, add the sugar and whisk for 1 minute.
1 cup sugar
Add the cocoa powder and stir until combined.
1/2 cup cocoa powder
Next, whisk in the egg and egg yolk as well as the vanilla.
1 egg + 1 egg yolk, 2 teaspoons vanilla
Stir in the flour, salt and chocolate chips just until combined.
1/2 cup flour, 1/4 teaspoon salt, 1/4 cup mini chocolate chips
Use a cookie scoop to scoop the batter into the mini muffin tin, filling each about 3/4 full.
Top with additional mini chocolate chips, if desired.
Bake for 10-12 minutes, or until the top is shiny and the center is still a bit soft.
Cool in the pan for 10 minutes then carefully remove to a cooling rack.