1. To prepare ricotta, line a large colander or sieve with 3 layers of dampened cheesecloth, allowing cheesecloth to extend over outside edges of colander, and place colander in a large bowl.
2. Combine milk and buttermilk in a large, heavy stockpot. Cook over medium heat until a candy thermometer reaches 170°, gently stirring constantly. As soon as the milk mixture reaches 170°, stop stirring (whey and curds will separate at this point). Continue to cook, without stirring, until the thermometer reaches 190°. (Be sure not to stir, or curds that have formed will break apart.) Immediately remove pan from heat. (Bottom of pan may be slightly scorched.) Pour milk mixture into cheesecloth-lined colander. Drain over bowl for 5 minutes, and discard liquid (whey). Gather edges of cheesecloth together; tie securely. Hang cheesecloth bundle from kitchen faucet, and drain 12 minutes or just until whey stops dripping. Scrape ricotta into a medium bowl. Sprinkle with salt, and toss gently with a fork to combine. Cool to room temperature.
3. Add 2/3 cup of granulated sugar, vanilla extract, and fromage blanc to ricotta; beat with a mixer at medium speed until combined. Cover mixture, and refrigerate.
4. To prepare shells, preheat oven to 375°.
5. Cut out 12 (12 x 4–inch) pieces of heavy-duty aluminum foil. Using your index fingers as a guide, loosely roll up each foil piece jelly-roll fashion to form a cylinder with a 1-inch opening. Lightly coat outside of each cylinder with cooking spray. Combine 1/3 cup granulated sugar and 1/2 teaspoon ground cinnamon in a small bowl.
6. Place 1 phyllo sheet on a large cutting board or work surface (cover the remaining dough to prevent drying); lightly brush with butter. Sprinkle evenly with 2 teaspoons sugar mixture. Repeat layers once. Cut phyllo stack lengthwise into 4 equal strips. Place a foil cylinder at bottom of 1 phyllo strip; roll up jelly-roll fashion around cylinder. Lightly coat with cooking spray. Place on a parchment paper–lined baking sheet. Repeat procedure with the remaining phyllo, butter, sugar mixture, and foil cylinders. Bake at 375° for 12 minutes or until lightly browned; cool completely on a wire rack. Carefully remove foil cylinders from phyllo shells by twisting ends of foil in opposite directions and gently pulling foil from shells.
7. Finely chop 1 1/2 ounces chocolate. Combine ricotta mixture and chopped chocolate in a bowl. Transfer mixture to a large zip-top plastic bag; snip off 1/2-inch of 1 corner of bag. Pipe ricotta mixture evenly into each of 12 prepared shells (about 1/3 cup). Grate remaining 1/2 ounce chocolate. Dust cannoli evenly with powdered sugar and grated chocolate; serve immediately.