Whisk 3/4 cup dark brown sugar, 1/2 cup granulated sugar, 2 teaspoons pure vanilla extract, and 1 teaspoon kosher salt into melted butter until smooth. Whisk in 1 large egg and 1 large egg yolk until smooth, about 3 seconds. Let the mixture sit for 3 minutes, then whisk for 30 seconds. Repeat process of resting and whisking 2 more times until mixture is thick, smooth, and shiny.