Preheat the oven to 425° F.

Brush melted butter into four 6-oz ramekins.

In a small bowl, combine 1 tablespoon cocoa powder with 1 tablespoon flour and use this to dust the insides of the ramekins, shaking out the excess.  Set aside on a baking sheet.

In a medium saucepan, combine the stick of butter with the chopped chocolate.  Heat over low heat until the butter and chocolate are melted.  Add 1 tablespoon of Nutella to the melted chocolate and stir in until incorporated.   Remove from the heat and stir in one teaspoon vanilla extract.

In a large bowl, beat the eggs and sugar and a pinch of salt with an electric mixer until thick and pale yellow, about 3 minutes.

Gently fold in the chocolate using a rubber spatula until incorporated.  Fold in the 1/4 cup of flour with the spatula, just until incorporated.

Spoon two-thirds of the batter into the prepared ramekins, then spoon one tablespoon of Nutella on top.  Cover with the remaining chocolate batter and smooth the tops.

Bake in the center of the oven for 16 minutes.  The tops will be cracked and the centers soft and jiggly.

Let the cakes cool for 6 to 8 minutes.

Run a knife along the edge of the cake and the ramekins to loosen.  Place the serving plate on top of the ramekin and invert the plate so that the cakes fall into the center of the plate.  Gently remove the ramekin.  Dust lightly with powdered sugar if desired.

Serve with vanilla ice cream, whipped cream or with fresh fruit, or all of the above.