Prep: Preheat oven to 375. Generously butter or spray non stick spray on 4 ramekins.
In a small saucepan, combine the butter and chocolate chips. Heat over medium-low heat, stirring constantly.
When shiny and smooth, remove from heat and allow to slightly cool.
Using a hand or stand mixer, combine eggs, egg yolks, and sugar. Beat for 5 minutes. Mixture will be pale. Fold in the chocolate mixture until completely combined.
Add in chocolate, flour, salt, and vanilla. Mix until completely combined.
Spoon enough batter into each ramekin so that it measures about an inch deep. Remove filling from freezer. It should be a fudgy consistency. Divide into 4, and with clean hands, roll quickly into balls. Place on top of batter. Spoon remaining batter over top of the filling balls.
Bake for 20-22 minutes until edges and top are firm, but center can still be jiggled. Remove from oven and allow to cool 10 minutes.
Run a knife around the edges of cake to release from ramekin. Invert cakes onto serving plates. Top with ice cream, chocolate sauce, or other desired toppings.