Toast the coconut: Preheat the oven to 300 degrees F. Line a baking sheet with parchment paper. Spread the coconut in an even layer on the baking sheet and toast, tossing and turning every 4 minutes or so, until the coconut just starts to turn golden, about 12 minutes total. Remove the coconut from the oven and set aside to cool.
Make the Cookie Base: Increase the oven temperature to 350 degrees F. Lightly spray a 9-by-13-inch baking pan with nonstick cooking spray and line it with parchment paper, allowing the parchment to overhang on two sides. Lightly spray the parchment paper.
In a medium bowl, whisk together the flour and salt.
In the bowl of a standing mixer fitted with the paddle attachment, beat the butter and sugar on medium speed until fluffy, approximately 2 minutes. Add the vanilla and the flour mixture and beat just until combined.
Turn the dough out into the prepared baking pan and, with lightly floured hands, press the dough into an even layer on the bottom of the pan (do not press up the sides). Prick the top of the dough with the tines of a fork and bake the crust for 25 to 30 minutes, rotating the pan halfway through the baking time, until golden. Remove the pan from the oven and place on a wire rack to cool.
Make the Caramel Layer: In a medium saucepan, combine the corn syrup, both sugars, and 2 tablespoons of water and stir gently so you don’t splash any of it up on the sides of the pan. Set the saucepan over low heat and continue to stir gently until the sugar dissolves. Clip a candy thermometer to the side of the pan, making sure the bulb of the thermometer is immersed in the syrup. Turn the heat up to medium-high and wait without stirring for the mixture to reach 240 to 245 degrees F, about 7 minutes. Keep a watchful eye on the temperature while you proceed with the next step—you do not want the mixture to exceed 250 degrees F.
Meanwhile, in a small saucepan over medium heat, stir together the cream and sweetened condensed milk. Gently warm the mixture; do not let it boil.
Once the sugar mixture turns amber, remove it from the heat and stir in the butter and the warm cream mixture until completely combined (be careful as it will bubble up when you add the cream mixture). Place the pan back over medium heat, stop stirring, and bring the mixture back to 245 to 250 degrees F.
Remove the pan from the heat, stir in the vanilla, and fold in 2 cups of the toasted coconut.
Pour the caramel directly onto the cookie base and, working quickly, spread it into an even layer with an offset spatula. Sprinkle the remaining 1 cup of toasted coconut over the caramel. Let cool to room temperature, about 2 hours, then place in the refrigerator to chill for at least 1 hour.
Line a baking sheet with parchment paper. Use a knife to loosen the chilled bars from the sides of the pan, then lift the bars out using the parchment paper overhang. Spray a sharp knife with nonstick cooking spray and cut the bars into twenty-four 3-by-1 1/2-inch rectangles and place on the prepared baking sheet. Refrigerate until ready to assemble.
Assemble the bars: Melt both chocolates together in the bowl of a double boiler over medium heat or in the microwave (in a microwave-safe bowl, cook the chocolate at 50% power (medium) for 15-seconds bursts, stirring between each blast, until melted).
Dip half of each individual bar in the melted chocolate, scrape off the excess chocolate with an offset spatula, and place the bar back on the baking sheet. Repeat with the remaining bars.
Alternatively, dip the bottom of each bar in the remaining melted chocolate, scrape off the excess chocolate with an offset spatula. Return to the baking sheet to set. Once set, squeeze the chocolate into one corner of the zippered plastic bag and twist the other end of the bag a few times to form a makeshift pastry bag. Use a kitchen scissor to snip off the small corner of the bag where the chocolate has collected (you don’t want too large a snip or the chocolate will ooze out).
Working quickly, drizzle the chocolate in a zigzag pattern across all of the bars. Place the pan of bars back into the refrigerator for 15 minutes to let the chocolate set up. Serve immediately. The bars can be stored in the refrigerator, tightly covered, for up to 4 days. They taste really good directly from the refrigerator as well as at room temperature—entirely up to you.