This is what I’ve used for making my stained glass window cakes. Because of it’s yellowish/brown tint any coloring you add will come out somewhat dull.
In a small sauce pan over medium heat melt:
1 1/2 c. salted butter
3/4 c. packed light brown sugar
3 tbs. milk
1 1/2 tbs. light corn syrup
Bring to a boil, stirring constantly. Boil and stir for 2 minutes and remove from heat. When cooled to room temperature stir in:
1 1/2 tsp. Vanilla Butter and Nut Flavoring (regular vanilla is also fine)
I cannot stress enough how important it is to keep stirring this while it’s cooking, if you don’t the butter will separate! Also, the longer you let this sit the more jelly-like it will become.
For decorating I’ll add some coloring and put it into a plastic disposable piping bag.