Preheat oven to 350°F (175°C). Grease a mini muffin tin well with cooking spray, then dust lightly with cocoa powder to prevent sticking.
In a saucepan, add melted (but not hot!) Butter, Sugar and Cocoa Powder. On Low-Med heat, continue to mix until smooth. (If the chocolate/sugar seizes up, add a tablespoon of cream to loosen it up. Repeat as required)
Once smooth, set aside to cool for 10 minutes
Add Eggs, Yolk, Vanilla and mix until incorporated and smooth.
Toss flour, Salt and Chocolate Chips in a bowl, then add to Cocoa mixture, mix until incorporated
Using a 2 TBSP Spoon, spoon the batter into each receptacle.
Bake for 16-22 minutes, till toothpick comes out with crumbs, not gooey batter.
Cool pan on a wire rack for 15 minutes minimum.
Using the tip of a knife, op out each mini-brownie and place on wire rack to cool completely