I feel like a food historian. LOL! I have Dad's recollection of ingredients.
Exhibit A: Butter, brown sugar, cornstarch, vanilla, cream, beaten egg yolks.
Exhibit B: Gramma's Aunt Eve Holden's actual recipe for 9" pie
1c brown sugar + one lump of butter, size of an egg
heat in pan until melted. Do Not Brown.
Mix together 1 c milk, 2 egg yolks & 2T flour: add to butter & sugar. Let boil, stirring constantly until thick.
Exhibit C: In 1848, the recipe for Butterscotch, the Royal confection, was 1 lb butter, 1 lb sugar and 1/4 lb treacle, boiled together. Treacle is molasses. Aha! A ratio of sorts!
Exhibit D: Brown sugar is a mixture of white sugar and molasses. 1 cup sugar + 1 T molasses. Want darker? 2 T.
Conclusion: Here is my rough draft recipe.
Measure a packed cup of brown sugar into your palm. Shape it into an egg. Using softened butter, make another 'egg' the same size as the sugar. Or weigh the sugar and get an equal weight of butter.
Melt in heavy saucepan. Meanwhile mix together 1 c cream, 2 egg yolks 1/2 t vanilla & 1T cornstarch. Add slowly to butter and sugar, stirring constantly until thick.
Do not try to make my conclusion. My research has a long way to go. I'll post when I get something better.