Lightly toast the hazelnuts in a preheated oven to 350F/180C for about 10 mins, until shiny and golden. Be sure to stir them every 3-4 minutes to keep them from burning. Once toasted, cool them off a bit before use.
Put the toasted hazelnuts in a food processor and pulse until crumbled. Keep blending the hazelnuts, in a matter of minutes they will reach a creamy but still dense buttery texture. To make a crunchy nutella, stop pulsing at this point. To make your Nutella smoother, keep processing the hazelnuts a little bit longer, for about 5 more mins, until you reach a shiny glossy texture.
Put chocolate, sugar and warm milk in a microwave-safe bowl and heat for 30 seconds only. Remove from the microwave and stir well until all the chocolate has melted and you have a glossy texture.Then pour the melted chocolate and milk mixture into the food processor with the hazelnuts.
Process all ingredients together for 1-2 mins, until the nutella gets shiny and smooth. Store in an airtight container in the refrigerator for up to one month.