Special equipment: 11- by 17-inch baking tray, lined with parchment paper
Break up 4 ounces of chocolate into 1/2-ounce pieces. Set aside.
In a food processor, finely grind the remaining chocolate with the sugar.
Lay 8 sheets of filo out on top of each other.
Place one sheet lengthwise on the work surface. Keep the remaining sheets covered with a damp towel.
Brush the sheet with some melted butter. Sprinkle with 1 1/2 tablespoons of the sugar mixture. Place a second sheet over the sugared sheet. Butter the second sheet and sprinkle with the sugar mixture. Add 2 more sheets, buttering and sprinkling between each.Cut the filo horizontally into 4 even strips. Starting at the left side of one of the strips, place a 1/2-ounce piece of chocolate 2 inches from the end. Fold the filo that is to the left of the piece of chocolate over the chocolate toward the back side of the strip, making a 3-inch triangle. The left end of the filo should be even with the back edge. Fold the piece of chocolate to the right, over itself, retaining the triangular shape. Fold the chocolate piece toward the front edge of the strip. Continue wrapping the chocolate, alternating the direction with each fold, until you get to the end. (It is like folding a flag.) Wrap the other strips in the same manner.
With the remaining sheets, make 4 more chocolate-filled triangles.
Preheat the oven to 350 degrees. Place the triangles on the baking sheet. Bake until golden brown, about 10 minutes. Serve warm.