Preheat the oven on 180 degrees Celsius or 356 degrees Fahrenheit.
Whisk egg just till mixed, and keep about 3 tablespoons in a separate bowl.
Mix butter, flour, sugar, the mixed egg & cacao powder until well combined.
Butter the oven dish and coat the oven dish with 2 tablespoons of flour. Move the oven dish till all the butter in the dish is covered with a little bit of flour.
Place the butter mixture in the oven dish & top off with the hazelnuts. Make sure you gently press the hazelnuts in the mixture, to make sure they don’t fall off when baked.
Brush the 3 tablespoon egg mixture over the mixture and bake for 25 minutes.