I substituted vanilla for grappa (admittedly that’s a lot of vanilla) with good results.

I also made a gluten-free version, substituting for the wheat flour a mixture of 6 parts brown rice flour to 2 parts potato starch (not potato flour) to 1 part tapioca flour, plus 3/4 tsp of xanthan gum mixed very thoroughly. I added an egg white to give the flour a little more support. The resulting batter wasn’t as crumbly as the wheat version, so I took care to break it into little pieces when adding the topping. It took 10 minutes longer to bake (with either version, I used the “bake it till it almost doesn’t jiggle in the middle” criterion.) The result was quite close to the wheat version, which sadly isn’t always the case in gluten-free baking.