Melt butter and chocolate in a microwave bowl for 1 min. 20 seconds. Whisk together till completely melted. Add powdered sugar, whisk till combined. Add eggs and yolk, whisk till combined. Add flour and lemon zest, whisk till combined.
Spray 4 custard cups liberally with non stick spray. Pour evenly into cups.
Bake for 13-15 minutes until sides are firm and center is soft.
Invert onto a plate, and serve with ice cream and enjoy!