Heat the oven to 225 degrees F. Line a baking sheet with parchment paper. Put the popcorn and toasted hazelnuts into a large bowl.
In a medium sauce pan, combine the brown sugar, olive oil, butter, corn syrup and salt; bring to a boil. Add the baking soda and cream of tarter; boil over moderate heat, stirring occasionally, until the syrup is a deep amber color, about 4 minutes. Pour the hot syrup over the popcorn and nuts; stir with a wooden spoon until evenly coated. Spread the mixture on the baking sheet and bake 1 hour. During the last 15 minutes of baking time; sprinkle the chopped Baci over the top of the popcorn and allow to melt.
Remove from the oven and allow to cool, the caramel corn will harden as it cools. Invert onto a cutting board, peel off the papers and break into chunks.
*To prepare the Hazelnuts: place the nuts in a single layer on a cookie sheet. Bake in a heated 350 degree oven for about 5 - 7 minutes, stirring once or twice, until lightly browned. Watch carefully so they don't burn. Or saute them in a dry skillet set over medium heat, stirring constantly, until lightly browned.
Variation: Substitute toasted slivered almonds or roasted and salted peanuts for the hazelnuts and use chopped Perugina Dark or Bitter-Sweet chocolate in place of the Baci.