Make a starter from the warm liquid, yeast, and 1 tbsp sugar, and let sit for 15 minutes for the yeast to activate.

Whisk the butter, egg, and sugar into a smooth mixture, add the starter, sift in 300 g flour, and add the salt and vanillin. Knead the dough, adding the remaining flour as needed. Shape into a ball and set in a warm place for 1-1.5 hours.

Strain full-fat sour cream (at least 20%) through a sieve in the fridge for 1-1.5 hours to drain the whey. Whisk the butter with the powdered sugar, add the strained sour cream, mix, and chill the frosting in the fridge.

For the filling, mix the sugar with the cocoa powder and cinnamon until smooth. Toast the sliced almonds in a dry pan.

Roll out the risen dough into a thin rectangle, spread with butter, and sprinkle with the sugar-cinnamon-cocoa mixture. Roll up into a log and cut into slices 3-4 cm wide.

Arrange the rolls on a parchment-lined baking tray, spaced slightly apart, and let proof for another 15 minutes. Bake for 20 minutes at 180°C.

Top the rolls with the frosting while they're still hot, and sprinkle with the nuts.