1. Prepare a heatproof glass measuring cup with nonstick cooking spray.
2. Heat the heavy cream in a small saucepan over medium heat until it comes to a boil, stirring frequently. Remove from heat and set aside. Cream will burn and boil over the pan if not paid close attention.
3. Combine the sugar with water (consistency should resemble wet sand) in medium heavy saucepan.
4. Heat, stirring constantly over medium-high heat until sugar has dissolved and is bubbling.
5. Stop stirring and let the sugar boil undisturbed until the sugar turns an amber color (360 degrees on a candy thermometer). This will happen in about 2-3 minutes. Be careful to not overheat the caramel or else it will taste very bitter. It is better to stop the caramelization before it gets too brown (a few degrees lower than 360) because the temperature will continue to rise from the heat of the pan.
6. Remove from heat and slowly pour the hot cream into the caramel. It will bubble and rise in the pan quickly, so be sure to stir vigorously.
7. Continue to stir the caramel over very low heat until the mixture is uniform in color and the caramel is completely dissolved.
8. Remove from heat and add the butter and mix until smooth.
9. Pour the caramel into the prepared glass measuring cup and allow it to cool for about 3 minutes.
10. Add the vanilla and salt.
11. Allow the butterscotch to cool to room temperature, stirring gently a few times.
12. Assemble cake by spreading a layer of Duff’s buttercream using an offset spatula followed by another layer of butterscotch sauce at your desired thickness onto one cake layer. Place the second cake layer on top of the filling-covered first.
13. Ice the entire assembled cake with Duff’s buttercream.