Thaw the phyllo overnight in the refrigerator. Then put the phyllo box on the counter to come to room temperature, 1-1/2 to 2 hours.
Make the filling: Place the hazelnuts, chocolate, sugar, and cinnamon in a food processor bowl fitted with the blade. Process until the nuts and chocolate are finely chopped (the largest should be the size of small dried lentils), about 15 to 30 seconds. Set aside.
Assemble the baklava: Unfold one pack of the phyllo sheets and stack them so that they lie flat on a work surface. Cover the top with plastic wrap, letting some excess plastic fall over all four edges. Pla
ce dampen kitchen towel on top of plastic wrap; this will hold the plastic in place and prevent the phyllo from drying out.
In a small sauce pan over medium heat melt butter. Brush bottom of a 9×13-inch metal pan (use a light color pan to prevent overbrowning to edges) with some of the butter. Remove a sheet of phyllo from the stack, (re-cover between using to keep phyllo moist and pliable) and place it on bottom of pan. Brush the sheet with some of the melted butter but don’t soak the phyllo (you’ll have about 40 layers of buttered phyllo once fully assembled). Repeat until 10 sheets have been layered and buttered. If pan has slightly angled sides, arrange sheets so the excess falls on the same side of the pan and cut the extra off every few layers with a paring knife. Sprinkle about one-third of the filling evenly over the phyllo.
Repeat layering and buttering remaining sheets from first pack and sprinkle on another third of the filling. Open, unfold, and cover second pack of phyllo. Layer and butter it as described above, sprinkling the remaining filling after layering about half the phyllo, and ending with a final layer of phyllo (you may not need all of the butter). Loosely cover and transfer pan of baklava to freezer for 30 minutes (this will make it easier to cut).
Bake the baklava: Position an oven rack in the center of the oven and heat the oven to 350°F.
Before baking, use a thin, sharp serrated knife and using a gentle sawing motion, cut the baklava to preferred serving portions. Avoid pressing down on the baklava while cutting it with the other. Bake the baklava until golden, 40 to 45 minutes. Transfer to a rack and let cool completely.
Make the syrup: In a small saucepan, bring sugar, espresso powder and 2/3 cup water to a simmer over medium heat, stirring occasionally, until the sugar is dissolved, about 5 minutes. Remove pan from heat and stir in Frangelico. Pour the syrup evenly over entire surface of the baklava, allowing it to run down into the cut marks and along the sides of the pan. Set baklava aside to cool to room temperature before serving.