We would recommend making your chocolate kagecreme ahead of time so that it has sufficient time to cool in the fridge. Heat up the milk, either in the microwave or in a saucepan, until it is "fingervarm" (finger warm), meaning it should feel warm to the touch but not hot. You don't want to kill the yeast! If using active dry or fresh yeast, add the yeast along with one teaspoon of the sugar in this recipe, and stir to dissolve. Let it rest 5-10 minutes until it begins to froth and bubble a little bit. (If your yeast doesn't do anything, it may be expired. Check the expiration date on the package and start over with new yeast!) If using instant yeast, you don't need to activate it, and you can add the milk and yeast directly to the egg mixture in the next step.
Combine the egg, egg yolk, remaining sugar, vanilla extract, and salt in a large bowl or the bowl of a stand mixer. Mix together until you have a homogenous mixture.
Add the milk and yeast mixture to the egg mixture and whisk to combine. Add in one shot of espresso as well, and mix to combine.
Add in the flour (reserving a small amount to start) and cocoa powder to the wet ingredients, stirring with a wooden spoon, or using the dough hook on your stand mixer for just about 30 seconds. Note: you may not need to add all of the flour, which is why we recommend reserving a small amount and gauging how dry the dough appears. You can always add more flour as you knead, but it's very difficult to get moisture back into the dough once it is too dry.
Once you have a scraggly dough, Add the softened butter and knead until smooth and elastic, probably around 10 minutes. Add a bit of flour if the dough sticks to your hands or the surface. If you prefer not to get your hands messy and covered in butter, you can knead the dough with a dough hook attachment in a stand mixer.
The texture of your dough before its first rise should be soft and smooth, not sticky and not too dry.
Cover and let your dough rise for at least 45 minutes, or until doubled in size.
Once the dough has risen, dump it out on a flat surface. Divide into 12 equal pieces.
Now we're going to shape the dough balls into buns. Take a piece, pinch together into a ball shape, then cup it in your hand with the top of the roll touching your palm. Place your hand on an unfloured surface with the seam touching the table. Move your cupped hand in quick, tight circles – this will cause the seam to come together and create tension on the surface of the dough.
Let the buns rest for about 30 minutes on the table, covered with plastic wrap or a damp dishtowel. I also like to spray or brush the tops of the dough with a bit of vegetable oil, just to make sure they don't form any skin or dry spots.
While the buns are resting, take your pastry cream out of the fridge. If it's very stiff, mix it together well with a spoon and it should have a nice texture!
Next, you are going to fill the buns with kagecreme or pastry cream, and any additional fillings you may like. To do this, take a bun, place it on your table or surface dome side down (seam side up) and flatten it, just by hitting it with the heel or palm of your hand. You may need to pick up the circle of dough and use your hands to enlarge the circle a bit, that's totally fine.
Either pipe or spoon a dollop of filling (a dollop on the smaller side; we can usually get a few teaspoons of filling in each - just be careful not to overfill, or else your buns will explode) into the center of the dough circle.
Pull up the sides and pinch them together. You do not need to do anything else to shape the buns at this point! This will only create weak spots where the filling can run out during baking. Place your filled bun on a baking sheet lined with parchment paper or a silicone baking mat.
Let the filled buns rise for another 30-45 minutes. In the meantime, preheat your oven to 350 F (175 C).
You can use the "poke" test to check whether your dough is done rising; if you poke the dough and it leaves an indentation and doesn't spring back immediately, it should be ready!
Before baking, egg wash the buns (an egg wash is just an egg beaten together with a little bit of water, then brushed onto the dough).
Bake the buns in a preheated oven for 12-14 minutes.
Let the buns cool on a wire rack. Once cool, prepare your decoration. Mix together about a cup of powdered sugar and a tiny bit of water. You can also add cocoa powder if you want it to be chocolate icing! Keep adding water, or sugar until the icing reaches the consistency and taste that you like.
Spoon the icing onto the top each bun. Add colorful sprinkles on top, let the icing harden a bit. You can also add leftover pastry cream to the top with sprinkles, or make a chocolate ganache. There are lots of customizations you can make, but we hope you enjoy!