This is the Vefa Bozacısı. From another site I take this text: “Vefa Bozacısı has been in business since 1876, and its founder Hacı Sadık Bey is the one who originated boza as we know it today. This historic shop sells only boza between October and April. During the warm months from April to October, you can find ice-cream, lemonade, and kuru üzüm şırası (a sweet, non-alcoholic drink made of fermented grape juice).

What is boza?
Bozais a nourishing and creamy drink, which is slightly sweet, served with a dusting of cinnamon on top. This thick and bubbly drink is made of fermented wheat; the grain is boiled in water, crushed, and drained. Sugar and a little bit of yeast are added and the mixture is left to ferment. Boza is ready to be drunk when it begins to bubble and has a slightly acidic taste, managing to be simultaneously sweet and sour. This sweet winter drink is traditionally enjoyed with leblebi (roasted chickpeas) in the evening.“

If you never tried it: do. It is wonderful.