Preheat the oven to 180°C (350°F) and line an 8"x8" baking pan with baking paper and set aside.
Beat the butter, brown sugar and sugar together until light and fluffy.
Beat in the egg and vanilla extract.
Beat in the cocoa, flour, baking soda and baking powder until all combined.
Press into the base of your prepared tin.
Spread over the Nutella (how much you use depends on personal taste) and sprinkle over the chocolate buttons.
Place the cocoa, crushed hazelnuts, flour, rolled oats and brown sugar together.
Pour over the melted butter and mix to a crumble texture.
Sprinkle over the crumble and place in the oven for 20-25 minutes.
*To roast the hazelnuts, heat the oven to 180°C and place the hazelnuts on a tray or in a large dish and place in the oven for around 10 minutes. The smell will be nutty and the hazelnuts will be slightly darker. Remove from the oven and allow them to cool before peeling the skins off and crushing them.