Place the hazelnuts in a baking pan. Bake at 200ºC for 10 minutes. Spread them on a towel and rub them to remove their skin. When they cool completely pop them into the food processor and pulse until they are ground. Melt the chocolate and butter together in a bain-marie. Remove from heat and add the sieved cocoa powder, milk and ground nuts. Combine everything very well.
Place the mixture inside a 400g jar while still warm. Set it aside to cool. Cover and keep it refrigerated.