Preheat oven to 350°F. Line 2 large baking sheets with parchment paper.
Spread the 4cup pecan halves in a single layer on one prepared baking sheet and toast in the oven for 6 minutes, shaking the pan once halfway through. Remove from oven and let cool for a few minutes.
On the lined baking sheets, arrange the toasted pecans into clusters of five pecan halves each, placing them in a star-shaped pattern with a small center well for the caramel. Leave about 1–2 inches between clusters for spreading. Set the sheets aside.
In a medium saucepan, melt the 1/2cup unsalted butter over medium heat. Add the 1cup granulated sugar, 1/2cup corn syrup, 3/4cup sweetened condensed milk, and 1/2teaspoon fine sea salt. Stir frequently and cook until the mixture reaches 235°F on a candy thermometer (soft-ball stage), about 8 minutes.
Remove the saucepan from the heat and vigorously whisk the caramel for 30 seconds. Let the caramel sit off the heat for 5 minutes.
Using a medium cookie scoop or spoon, drop level mounds of the caramel into the center of each pecan cluster.
While the caramel rests, set up a double boiler: fill a medium pot one-third full with water and bring to a low simmer over medium heat. Place a heatproof bowl on top so it does not touch the simmering water. Reduce heat to low. Add the 8ounces dark chocolate (roughly chopped) to the bowl and stir occasionally with a spatula until completely melted and smooth. Remove the bowl from the heat.
Spoon about a tablespoon of the melted chocolate over the caramel on each pecan cluster, centering the chocolate over the caramel. If using, sprinkle flaky sea salt on top while the chocolate is still wet.
Let the turtles sit at room temperature until the chocolate has set, about 1 hour. Store in an airtight container at room temperature for up to 1 week.