Combine 1 cup sweetened condensed milk, 6 tablespoons unsalted butter, and 2/3 cup granulated sugar in a medium saucepan over medium heat and stir constantly until the butter melts and sugar dissolves, about 3 to 4 minutes.

In a small bowl, lightly beat 3 egg yolks with a fork, then temper them by slowly whisking in about 1/4 cup of the hot saucepan mixture to prevent scrambling.

Pour the tempered egg mixture back into the saucepan and cook over medium heat, stirring frequently, until the mixture reaches 160°F or thickens to coat the back of a spoon, about 5 to 7 minutes.

Remove the saucepan from heat and stir in 1 teaspoon vanilla extract, then let the frosting cool for 2 to 3 minutes.

Fold in 1 1/3 cups shredded coconut gently until evenly distributed, then fold in 1 cup chopped pecans and 1/4 teaspoon salt.

Let the frosting cool at room temperature for 5 to 10 minutes, stirring occasionally, until spreadable. If it becomes too thick, warm gently over low heat for 30 seconds before using.