Combine equal parts granulated sugar and water in a saucepan. Heat until the sugar dissolves, approximately 5 minutes. Cool and refrigerate.

In a stand mixer, add warm whole milk, sour cream, granulated sugar, a beaten large egg, and fresh yeast. Mix at low speed, then add the all-purpose flour gradually. Continue mixing until a rough mass forms, about 2-3 minutes.

Add the salt and softened unsalted butter. Increase speed to medium and knead for about 10 minutes until smooth and elastic. Test for readiness with the windowpane test.

Transfer the dough to a lightly greased bowl. Cover and let rise in a warm area for about 1 hour until doubled in size.

Melt bittersweet chocolate and unsalted butter in a small saucepan over low heat. Stir in cocoa powder, powdered sugar, and vegetable oil until smooth.

Punch down the dough, divide in half, and roll into a rectangle. Spread cocoa filling over one, roll tightly and seal edges, then repeat with Nutella on the second half.

Cut rolled dough into strips, twist them, and place them in a greased baking pan. Cover and let rise for about 35 minutes until puffed.

Preheat oven to 170°C (340°F). Bake for 45 minutes until golden brown.

Brush the warm cake with the cold syrup prepared earlier. Let cool in the pan before serving.