I used the Highland Boundary larch and honeysuckle spirit, as I didn't happen to have their birch one. It's magnificent - probably a different cocktail with the slight conifer tang of the larch, but it really does work with this recipe.
Made this with Birch/Elderberry eau-de-vie. No idea how this compares to commercial Birch spirits, none of which I can find locally. Just acquired the Italicus Bergamotto and love it. Great combination.