Found a recipe at this webpage, it's pretty simple:
8 tbsp butter (1 stick)
½ cup cocoa
¾ cup sugar
¾ cup Nutella
2 oz. chocolate (dark works well)-optional
½ cup flour
½ tsp salt
Preheat the oven to 350. Grease an 8×8 inch pan.
Melt the butter in a saucepan over low heat, then add the cocoa and whisk together. When it’s smooth, add the sugar, Nutella and optional chocolate. Whisk in the eggs. Sift the flour and salt together in a separate bowl. Add the flour mixture to the wet ingredients and fold together. Pour into the greased pan. Bake for 20 to 25 minutes. Cool and slice.
I love the flavor, but I might have added a bit too much Nutella-- they were QUITE fudgy. Maybe next time I'll up the flour ratio just a bit.
Actually, next time I might try Nutella cookies or gelato!