The exact recipe for Almond Brandy is a closely guarded secret of the Moonshae Isles. According to one tabaxi thief who came close to stealing the recipe and barely escaped the islands with her life, the fundamentals include sweet almond liqueur blended with strong grape brandy, cut with a little lime for sharpness and a dash of bitters for depth.
Given the importance that Madame Rosene places on authenticity for everything in her museum, her adoption of this recipe in the Driftwood Tavern might be an endorsement of its authenticity. It's also quite likely that the drink sells well to tourists and that regulars also enjoy the bittersweet flavor!