Thoroughly grease the insides of two (4 oz) ramekins with butter. Make sure you don't miss any spots, especially along the bottom of the ramekin, otherwise the cakes won't pop out properly. Lightly flour the inside of the ramekins. Turn ramekins upside down and hit a few times against counter so that any excess flour falls out.

Add Nutella to a large mixing bowl. Warm in microwave for approximately 30-50 seconds. Add in egg mixture and flour. Using a spatula, gently fold egg and flour into the Nutella until ingredients are mixed and batter is smooth and uniform.

Pour batter evenly into the two ramekins. Place ramekins on a baking sheet and place into the middle section of the oven (do not place too high or too low of a position in the oven because it may lead to uneven cooking). Bake for approximately 11 minutes. Watch carefully. The top of the cakes should be just cooked but the middle of the cake is not completely set.

Let cake cool in ramekins for about 3 minutes. Use a spatula and run around the edges of the cake to loosen the cake. Place your dessert plate, inverted, on top of ramekin. Pressing the plate on top of the ramekin (with gloves as the ramekin should still be quite hot), very carefully and quickly, turn over so that the plate is now at the bottom and the ramekin is upside down. (You need to do this in one fluid motion, which may take some practice at first. Otherwise, the liquid lava filling will tumble out too quickly and break through the bottom layer of the cake. It's still edible, but not quite as pretty.) Gently tap the upside down ramekin to ensure cake has popped into plate. Lift up ramekin. Serve with fruit, ice cream, chocolate sauce, etc.