Make the caramel: In a medium saucepan, combine the butter, sugar, corn syrup, and sweetened condensed milk. Cook over medium heat, stirring constantly so the caramel doesn’t scorch on the bottom.
Bring the mixture to a gentle boil and continue cooking until it becomes a beautiful light golden caramel color, about 8–10 minutes. For the most accurate results, use a candy thermometer and cook the caramel until it reaches 248°F, or firm-ball stage.
Cool the caramel: Carefully pour the hot caramel onto a parchment-lined baking sheet. Spread it into an even layer with a spatula and allow it to cool at room temperature for about 1–2 hours, or until it is soft and chewy but firm enough to handle.
Prepare the pecans and chocolate: Melt the chocolate in the microwave in 30-second intervals, stirring well after each interval until completely smooth. You can also melt it over a double boiler.
Line another tray with parchment paper and arrange the pecan halves in small clusters, using about four pecans for each turtle and positioning them so they resemble little turtle legs.
Assemble the turtles: Place a small spoonful of melted chocolate in the center of each pecan cluster. This helps hold the pecans together.
Cut or roll the cooled caramel into small pieces and place one in the center of each cluster. Gently press it down, then spoon melted chocolate over the caramel, allowing some of the pecans to remain visible around the edges.
Let them set: Refrigerate the turtles for 1–2 hours, or until the chocolate is completely firm. Once set, transfer them to an airtight container and store at room temperature for a softer caramel or in the refrigerator for a firmer texture.