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.
4 ounces semi-sweet baking chocolate, ½ cup butter
Whisk together butter and chocolate until chocolate has melted and is incorporated.
Add the powdered sugar, and whisk until combined.
1 cup powdered sugar
Whisk in the eggs, yolks, and vanilla until well-combined.
2 large eggs, 2 egg yolks from large eggs, 1 teaspoon vanilla extract
Add flour, and whisk until just combined.
6 tablespoons all-purpose flour
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.
Notes
Chocolate: I recommend using a baking chocolate bar, like Baker’s Chocolate or Ghirardelli. Chocolate chips may not melt as well, and you may end up with grainy chocolate.
Butter: I like to use salted butter in this recipe. You can use unsalted butter with a pinch of salt if you prefer.
Powdered sugar: Using powdered sugar in this cake helps to give it its tender, fudgy texture.