Before starting: Make sure your butter is fully softened. If it’s not, cut it into small pieces and let it sit on the counter for 10 minutes. The dough will not come together nicely if the butter isn't soft. Do not melt the butter either.
In a small bowl, place dry yeast, warm milk and the tsp sugar. Let sit.
In the bowl of a mixer, add 4 cups of flour (leaving 1 cup for later). Add salt, sugar, vanilla sugar, eggs, softened butter, and sour cream. Add the yeast mixture. Mix on medium speed for 3 minutes. Slowly add the additional cup of flour and mix for an additional 2-3 minutes.
Dough should be slightly sticky but manageable. If you feel the dough is too sticky add up to ½ cup of flour. (The less flour, the lighter the dough will be when baked). Use a large spatula to remove the dough from the bowl and place it into a greased bowl. Cover with plastic and let rise for 1 hour.
Preheat oven to 350°F.
On a clean work surface (see this hack to keep your kitchen clean) Divide the dough into 4 parts. Roll the dough tightly into 4 balls. Use a little flour if needed.
Roll out each dough into a 9 x 13-inch rectangle. Flip it over and roll out larger into an approximately 13 x 18 inch rectangle.
Spread ¾ cup cocoa mixture on each rolled out dough. Break up ¼ of a stick (1 oz/2 tbsp) of butter onto each dough, spreading it out. Drizzle 3 tbsp of oil on each one.
Roll up each dough from the shorter end so that you have a shorter dough length and more layers in the roll. You want to gently pull as you are rolling, tucking in the ends so that the chocolate doesn't ooze out when baking.
Place rolls on a lined baking sheet.
Combine egg with sugar. Brush tops of rolls. Bake for 30 minutes or until ready and golden on top; this will take a little longer in some ovens.
The kokosh cake will keep out on the counter in a container for a few days or freeze and remove as needed.