Preheat the oven to 350°F (175°C). Spread the pecan halves in a single layer on a baking sheet and toast for 6 minutes, shaking the pan halfway through. Remove and let cool for a few minutes.
Line two large baking sheets with parchment paper and lightly spray or grease the parchment. Arrange the toasted pecans on the sheets in clusters of about five pecan halves each (star-shaped), leaving space between clusters.
In a medium saucepan over medium heat, melt the unsalted butter. Add granulated sugar, corn syrup, sweetened condensed milk, and 1/2 teaspoon fine sea salt. Stir to combine.
Cook the mixture, stirring frequently, until it reaches 235°F (soft-ball stage) on a candy thermometer, about 8 minutes. Remove from heat and whisk vigorously for 30 seconds, then let the caramel rest for 5 minutes.
Use a medium cookie scoop to drop level mounds of caramel (about 1 scoop each) onto the center of each pecan cluster. Allow the caramel-topped clusters to rest while you melt the chocolate.
Set up a double boiler: fill a medium pot about one-third full with water and bring to a low simmer. Place a heatproof bowl over the pot (not touching the water) and add the chopped dark chocolate. Reduce heat to low and stir until fully melted and smooth, then remove the bowl from the heat.
Spoon about a tablespoon of melted dark chocolate over the center of each caramel pecan cluster, covering the caramel. Sprinkle a little flaky sea salt on top of any pieces you want salted.
Let the turtles set at room temperature until the chocolate is firm, about 1 hour. Once set, transfer to an airtight container and store at room temperature.