1. Preheat oven to 350°. Line a baking sheet with parchment paper and set aside.
2. On a clean cutting board, place one piece of phyllo dough and spray generously with cooking spray. Add another layer of the phyllo and repeat; finish all four sheets. Trim the layered phyllo dough into a square approximately 13x13 inches.
3. Roll or fold up the sides of the square to form a 9-inch circle with a 1/4-inch folded lip. Place the shell onto the prepared baking sheet. Prick the bottom of the tart all over with a fork.
4. Bake phyllo until golden brown (about 15 minutes); remove and allow to cool. (Tart shell can be made a day ahead and stored in an airtight container.)
5. Cook sugar in a small, heavy dry saucepan over medium-high heat, undisturbed, until it begins to melt (2-3 minutes). Continue to cook another 1-2 minutes, stirring occasionally, until sugar is melted into a deep golden caramel.
6. Remove from heat and carefully pour in cream (mixture will steam and bubble). Once bubbles begin to subside, return pan to moderate heat and cook, stirring constantly, until caramel is dissolved; remove from heat and add chocolate and salt; stir until chocolate is melted. Cool slightly before pouring mixture onto crust; sprinkle with extra sea salt flakes, if desired. Chill until filling sets (up to 30 minutes). Slice into 10 wedges and serve.