Instructions

Adjust oven rack to upper-middle position and heatoven to 350 degrees. Spray 9-inch springform pan and ovensafe 2-inch round cutter with vegetable oil spray.

for the crust

Combine flour, sugar, and salt in bowl. Add melted butter and vanilla; stir with silicone spatula until evenly moistened. Crumble dough over bottom of prepared pan. Using bottom of dry measuring cup, press dough into even layer. Using fork, poke dough all over.
Press 2-inch round cutter into center of dough. Keeping cutter in place and using small spoon or offset spatula, gently remove center round of dough; discard or bake separately. Bake crust until edges are light golden brown, about 20 minutes. Using bottom of dry measuring cup, gently press on entire surface of warm crust to compress. Let crust cool for at least 20 minutes.

for the filling

Meanwhile, combine sugar, condensed milk, cream, corn syrup, butter, and salt in small heavy-bottomed saucepan. Cook over medium-low heat, stirring frequently with silicone spatula, until mixture registers between 236 and 239 degrees (temperature will fluctuate), 16 to 20 minutes. Off heat, stir in vanilla and 1 cup coconut. Pour caramel mixture over crust and, using silicone spatula, spread to even thickness (mixture will be very hot). Sprinkle evenly with remaining ½ cup coconut. Let cool completely, at least 1½ hours. (Cookie can be loosely wrapped in plastic wrap and stored at room temperature for up to 24 hours.)

for the chocolate

Microwave chopped chocolate in liquid measuring cup at 50 percent power, stirring every 15 seconds, until melted but not much warmer than body temperature (check by holding in palm of your hand), 1 to 2 minutes. Add grated chocolate and stir until smooth, returning to microwave for no more than 5 seconds at a time to finish melting if necessary.
Remove side of pan and cutter in center of dough; invert cookie onto inverted rimmed baking sheet. Pour ¼ cup melted chocolate onto cookie in even band around hole in center, about ½ inch from hole. Using small offset spatula, gently spread chocolate to completely cover cookie. Repeat with additional ¼ cup chocolate. Refrigerate until chocolate is set, 20 to 25 minutes.
If remaining ½ cup chocolate has hardened, microwave until just remelted, stirring every 10 seconds. Transfer chocolate to pastry bag or small zipper-lock bag and snip off corner, making about ⅛-inch hole. Invert cookie, chocolate side down, onto wire rack. Carefully place rack in now-empty rimmed baking sheet. Pipe remaining chocolate in zigzags across caramel layer. Refrigerate until set, about 10 minutes. Using large offset spatula, transfer cookie to cutting board. Using chef's knife, cut cookie into 2-inch wedges. Serve. (Cookies can be stored in airtight container at room temperature for 4 days.)