Preheat oven to 425 F. Grease and flour 4 ramekins, and set them to a rimmed cookie sheet.
Using a mixer (either hand or stand), beat yolks and eggs until pale yellow, about 3 minutes.
Add flour, and beat until well-combined.
Stir in Nutella and vanilla.
Divide batter evenly among ramekins. Check out the note below if you want an extra molten center.
Place the ramekins on a rimmed baking sheet with at least 1-2 inches in between the ramekins.
Bake cakes until edges are set but the centers is still jiggly, about 12–14 minutes. If you overbake the cakes, then they won't have much molten Nutella in the middle.
Transfer cookie sheet to a wire cooling rack.
Let cakes cool for 5 minutes.
Run a thin knife around edges of ramekins to loosen cakes, and invert cakes onto plates.
Nutella: Or other chocolate hazelnut spread.
Ramekins: I use 6 ounce ramekins.
Molten center: If you want an extra molten center, pour half of the batter into your prepared ramekin, drop up to 1 teaspoon of Nutella in the center of the batter. Cover with remaining batter. Please do not use more than 1 teaspoon of Nutella as it will cause the cakes to brake apart when you invert them.
Make-ahead:After step 5, the ramekins can be covered and refrigerated for up to 24 hours before baking. Uncover and bake as-directed. You may find that you need to add an additional 1-2 minutes to the baking time.