Butterscotch with frosting

6 large eggs 1 1/2 cups dark brown sugar , firmly packed 1 teaspoon pure vanilla extract 1/4 teaspoon salt 1 1/2 lbs unsalted butter , cold 2 tablespoons[...]

3/4 cup butter , melted 2 cups light brown sugar , packed 3 eggs 2 cups flour 2 teaspoons baking powder 1/2 teaspoon salt 1 teaspoon vanilla extract 1 cup[...]