Chill your jar of Nutella for 45 minutes prior to making this recipe so that it's easy to scoop. 

Grease four 10-oz oven safe ramekins with butter and dust with cocoa powder and place on a small baking sheet.

In a medium saucepan over very low heat, gently melt the chocolate and diced butter together, stirring occasionally, until chocolate and butter are completely melted. Set aside for a couple of minutes to cool slightly. Then stir in the salt, flour and cocoa powder.

In a medium bowl with a hand mixer, or the bowl of a stand mixer fitted with the whisk attachment, whisk the eggs on high. Slowly stream in the sugar and whip on high until the eggs are light and fluffy. Fold the in the chocolate mixture until no streaks remain. Divide evenly between the ramekins. Gently press one tablespoon of chilled Nutella into the center of each cake. Chill for at least 30 minutes, up to 5 hours. 

Preheat an oven to 350 F. Bake for 35-40 minutes until the tops have cracked slightly. Remove from the oven and cool slightly. Serve dusted with powdered sugar and a scoop of ice cream if desired.