Grind the roasted hazelnuts into small pieces. Melt the dark chocolate, mix with the sunflower oil and hazelnuts. Remove the mascarpone bar from the blister. Place on a tray and insert 2 skewers into the bars. Dip half of the bar in the chocolate. Put them back on a baking tray and cover with plastic. Thaw in the fridge, or store in the freezer. Remove the skewers. Whip the cream until stiff and add the hazelnut paste. Pour into a piping bag with a St.-Honoré spout and decorate the bars.