Pre-heat the oven to 350 degrees
Combine the stick of butter and the chocolate in a small saucepan over very low heat, stirring occasionally
Remove saucepan from heat, add flour and stir until well mixed
Add two eggs and mix well, add additional egg yolk and mix until well incorporated
Add Kosher salt and pure vanilla extract
Spoon brownie batter into cupcake cups about 3/4 of the way
Bake for 15 - 20 minutes until cooked through, test with toothpick