Melt butterscotch chips and butter slowly in a double boiler. Remove from heat and let cool 5 minutes. Whisk yolks into butterscotch mixture until fully incorporated. Set aside. 

In a stand mixer, whip egg whites, cream of tartar and first sugar until stiff peaks form. 

Next, in a stand mixer, whip heavy cream, vanilla extract and second sugar until stiff whipped-cream consistency. 

Fold egg white meringue and whipped cream into butterscotch mixture until fully incorporated. 

 If mixture isn’t to your desired thickness, chill, stirring occasionally, until desired consistency. Fill serving dishes and top with a whipped cream or toasted marshmallow fluff and crushed-up pretzels.