Make up the pastry using the rubbing in method, adding in enough water to bind together well. Chill for 15 minutes then use to line a 9-10inch round fluted flan tin. Bake blind in the oven at 200°C for 20-25 minutes. At the same time, toasted the 150g hazelnuts in the oven. When ready, remove from the oven and immediately remove any skins and blitz the nuts in a processor to a butter. Add in the melted white chocolate and blitz together to a smooth paste. Spread this mixture over the flan base and spread out. Next, cover the top with the 100g toasted hazelnuts and place into the fridge to chill. Meanwhile, draw a circle onto a piece of nonstick paper the same size as the flan. Pour over the chocolate and spread out into the circle. Chill to harden then place the chocolate circle over the flan. Using a hairdryer, melt the chocolate disk over the nuts to take their shape.