Toast the hazelnuts: Lay hazelnuts on a cookie sheet and toast at 350 degrees F for 10-15 minutes until lightly browned and skins are beginning to crack. Wrap in a towl and let steam for 1 minute. Rub hazelnuts in the towel to remove some of the skins. Grind coarsely and let cool. Keep the oven at 350.
Make the crust: Combine all crust ingredients except the butter in a food processor until well mixed. Cut the butter into cubes and incorporate into other ingredients for about 1 minute. Press firmly onto bottom of 9-inch springform pan (I use bottom of flat glass). Bake at 350 for 10 minutes.
Make the cheesecake: Increase oven temperature to 400 degrees and place a pan half-filled with water in the bottom rack. Melt the chocolate and set aside. (Tip: if melting in the microwave, heat in intervals of 30 seconds and stir. Should take no more than 1 minute on high). On low speed, beat cream cheese until smooth. Slowly mix in the sugar, flour and frangelico. Keep scraping down the sides of the bowl. One at a time, beat in the eggs on low speed.
Take 1/2 cup of the cheesecake batter and mix it with the chocolate.
Pour cheesecake batter into pan. The chocolate mixture can either be placed in a bag for piping or dropped by spoonfuls into the batter and swirled for a marbled effect.
Bake at 400 degrees for 10 minutes. Decrease oven temperature and bake at 225 degrees for about 1 hour. Towards the end of the baking time, check for doneness. Move the cheesecake pan slightly -- if the edges are set and the center is still wobbly the cheesecake is done. Turn the oven off and prop the door open with an oven mitt. Let cool inside the oven for at least an hour. Let cool on a counter until the pan is cool. Place in the refrigerator uncovered to chill and set. The cheesecake must be chilled for at least 24 hours.