Preheat oven to 350°F. Grease two standard size muffin cups with cooking spray
In a microwave-safe bowl, combine the butter and sugar. Microwave for 30 seconds or so until butter is melted, then stir with a fork to well combine. It should be slightly lighter in color.
Add the egg yolk and vanilla extract then stir with the fork until combined.
Add the cocoa powder, flour, and salt then stir to combine using a rubber spatula. Fold in the chocolate chips.
Divide the mixture between the two greased muffin cups and bake for 16 - 18 minutes until the edges are set and a toothpick comes out with fudge brownie but not a lot of wet batter.
Allow to cool and enjoy!
