Preheat oven to 175c | 350° F. Lightly grease a 9" cast iron skillet (or pie dish) with butter/oil, wipe over excess and set aside.

Combine the butter and chocolate in your cast iron pan/skillet, and melt over low heat. Allow to cool slightly and pour the mixture into a heat proof bowl. Add the sugar and whisk until light and creamy. Add the eggs - one at a time - whisking between each egg, until combined.

Add the flour, cocoa powder and salt, mixing with a wooden spoon until just combined. Fold in the crushed hazelnuts; pour 1/2 the cookie dough into the prepared pan.

Melt the nutella in the microwave for 30 seconds OR in a heat-proof dish over a pot of boiling water until it reaches a thinner consistency.

Pour the nutella over the cookie dough in the skillet evenly to 1-inch of the edges. Sprinkle the wafers and whole hazelnuts over the dough. Top with remaining cookie dough, smoothing the layer with the back of a metal spoon.

Bake in preheated oven for 25-30 minutes (for a gooey cookie). Or for a set cookie, leave it in the oven for 10 minutes longer. Cool for about 10 minutes. Top with (optional) extra rushed hazelnuts and Ferrero Rochers.