Prepare the Syrup: In a medium saucepan, combine 1 cup granulated sugar and 1 cup water over medium heat. Stir until the sugar is completely dissolved, about 5 minutes, then remove from heat and allow to cool. Once cooled, refrigerate the syrup while you prepare the cake.

Make the Dough: In the bowl of a stand mixer, combine warmed milk, sour cream, sugar, egg, and fresh yeast. Gradually add all-purpose flour and salt, mixing on low speed. After the mixture comes together, add softened butter and knead for about 10 minutes until smooth.

First Rise: Cover the dough with plastic wrap or a kitchen towel. Place it in a warm area and let it rise for approximately 1 hour until doubled in size.

Prepare the Filling: Melt bittersweet chocolate and unsalted butter in a saucepan over low heat. Once melted, remove from heat and whisk in cocoa powder, powdered sugar, and vegetable oil until silky.

Assemble Cake: Once the dough has risen, punch it down, turn it out onto a floured surface, and roll into rectangles. Spread the cocoa filling and Nutella, roll tightly, and cut into strips.

Second Rise: Place the shaped dough in a greased baking pan, loosely cover, and let rise for another 35 minutes.

Bake: Preheat the oven to 170°C (340°F), bake for 45 minutes or until golden brown and a skewer inserted comes out clean.

Glaze: Brush the chilled syrup generously over the warm cake for a shiny finish.