Basic recipe for homemade Kahlua on the Intertubes is lots of sugar, some coffee, high alcohol content something or other, and vanilla. Some minor variations on this theme is all I've found.
But real Kahlua is heavier than Irish Cream. The layered shot, "Duck Fart," is made of, from bottom to top, Kahlua, Irish Cream, and Whiskey. When I try to make this with homemade Kahlua though the cream sinks.
The amount of sugar in this stuff is huge, it's basically syrup. I doubt it needs more. I use brown sugar. What could be missing from the recipe that makes it heavier than cream?