(This post is based on visit's to the bakery's previous location, in Bath Beach.) When in doubt, assume walnuts.
Custom cakes are the stock in trade of this "Russian-European style bakery." But "Baku" points to the inspiration for the display case of ready-made treasures: the capital of Azerbaijan, a country in the Caucasus that borders Russian, Georgia, Armenia, and Iran as well as the Caspian Sea. A sheker bura (first photo, about $2) might be filled with ground, sugared almonds or hazelnuts, but walnuts seem to be the default. Since sheker buralar (that's my best shot at the plural) are associated with Novruz, a holiday that celebrates the arrival of spring and is also referred to as the Persian New Year, perhaps the advent of warmer weather will herald a choice of fillings, too.
Mutaki (at the time, $7.50 per pound), which had a similar filling and a firmer shell, are handier to carry out and to share. Baku Bakery packages these pastries commercially for sale in local markets; the label, I noted, mentions almond flour and cardamom, as well as sour cream, eggs, and sugar, among the ingredients. Also shown: gata ($7.99 per pound). Though unlabeled, these typically sport plenty of sour cream, eggs, and sugar themselves.
Baku Bakery
1903 Ave. U (East 19th-Ocean Aves.), Sheepshead Bay, Brooklyn
718-996-5414
www.BakuBakery.com