Heat 1 1/2 cups brown sugar, 1/2 cup evaporated milk, 4 tablespoons unsalted butter, and salt in a large saucepan over medium heat until small bubbles appear around perimeter of the pan, around 4 to 8 minutes. Reduce heat to low and simmer, stirring occasionally, until large bubbles form and the mixture has thickened and turned deep golden brown, about 6 minutes; transfer to a large bowl. Stir in remaining 1/2 cup evaporated milk and remaining 4 tablespoons unsalted butter until mixture is slightly cool. Add 8 ounces chocolate and 1 teaspoon vanilla extract and stir until smooth. Whisk in 3 cups confectioners' sugar until incorporated. Let cool for 15 to 20 minutes, stirring occasionally.