Meanwhile, whisk egg yolks in a large bowl until pale yellow, about 5 minutes. Whisking constantly, slowly pour about 1 cup of the caramel sauce into the eggs. Pour the egg mixture back into the saucepan with the remaining caramel sauce, stir in salt, and cook over low heat, stirring constantly until it is as viscous as melted ice cream and coats the back of a spoon, about 5 minutes. (When you draw your finger across the spoon, it should make a mark through the custard, which should not run back in on itself.)