Preheat oven to 425 F. Grease 4 ramekins. I use 6 ounce ramekins.
In a large, microwave-safe bowl, microwave chocolate and butter on high for 1 minute, or until butter has melted.
Whisk together butter and chocolate until chocolate has melted and is incorporated.
Add the powdered sugar, and whisk until combined.
Whisk in the eggs, yolks, and vanilla until well-combined.
Add flour, and whisk until just combined.
Pour batter into prepared ramekins.
Set ramekins on a rimmed baking sheet.
Bake for 12-14 minutes, or until the edges are firm but the center is still soft.
Let the cakes stand for 1 minute.
Run a thin knife around the edge to loosen cakes, and invert onto plate.
Dust with additional powdered sugar, if desired, and serve warm.
To make ahead: Just follow the recipe through step 7. Then, cover with plastic wrap and place the ramekins in the fridge until you're ready to bake them. Set the ramekins out while you preheat the oven, uncover, and bake as directed.