Heat the oven to 325°F. Place the pecans on a rimmed baking sheet and roast until browned and fragrant, about 12 minutes. Remove from the oven and set aside to cool; once completely cooled, place the nuts in a gallon-size resealable plastic bag and slightly crush with a rolling pin to make smaller pieces. Set aside.
Spray an 8-inch-square baking dish with nonstick cooking spray. Fit with a piece of parchment paper long enough to fit into the bottom of the dish and hang over two opposite sides (creating parchment handle’s), and set the dish aside.
Place the cream, corn syrup, both sugars, and the butter in a medium saucepan over low heat. Stir often until the butter and sugar melt, using a heatproof spatula to scrape down the sides of the pan occasionally. Increase the heat to medium and continue to simmer the mixture until its temperature reads 244°F on a digital thermometer, 12 to 15 minutes (be careful-once the sugar hits 240°F, the temperature climbs rapidly, so stay on your toes!). If the mixture begins to bubble up to the rim of the pan, remove it from the stovetop for a second to calm the bubbles, give it a stir, and place it back on the burner. Once the caramel reaches 246°F, remove it from the heat and stir in the vanilla, salt, and pecans. Pour the mixture into the prepared baking dish and set aside in a cool spot (or refrigerator) for at least 4 hours or overnight.
Pour water into a small pot to a depth of 1 inch and simmer. Place the chocolate in a heatproof medium bowl and set it over the simmering water (the bottom of the bowl shouldn’t touch the water). Reduce the heat to low and melt the chocolate, stirring often, until it’s completely smooth. Pour the chocolate into a small bowl or ramekin and set aside until completely cool but still soft enough to dunk the turtles.
Run a paring knife around the edges of the baking dish and pop out the caramel block. Move to a cutting board and slice the caramel into squares between 1¼ and 1½ inches in size. Place a long sheet of parchment or waxed paper on your work surface. Dip each caramel square halfway into the chocolate, and then place on the paper to set up. After dipping, give the chocolate enough time to set up and harden, about 1 hour. Store the turtles in an airtight container in the refrigerator for up to 1 week.