Melt the butter and sugar in a heat proof bowl in the microwave or on the stove top in a small saucepan until the mixture starts to bubble. Let the mixture cool for at least 5-10 minutes. Stir in the 2/3 cup of Nutella, vanilla and vegetable oil.In a medium mixing bowl or in the bowl of a stand mixer, add the eggs. Beat the eggs for at least 3-5 minutes until they are fluffy and paler in color. They can be beaten up to 10 minutes. Add the beaten eggs to the Nutella mixture and stir until combined. Add cocoa powder, salt and flour and mix until incorporated. The mixture will be very thick at this point. Pour in boiling water or coffee and whisk until the batter is smooth. Pour into a parchment lined 8 inch square baking pan.With the remaining 1/2 of Nutella, use a teaspoon to add spoonfuls of Nutella around the top of the batter. Use a knife to gently swirl the Nutella into the batter. Do not over-swirl.Bake at 350 for 25-28 minutes. Cool on a wire rack. Refrigerate for at least 2 hours or overnight. Cut into 16 squares.