Heat the milk on low until it is warm to the touch - not too warm as it will kill the yeast.
Meanwhile place the yeast and sugar in bowl, stir to mix.
Once the milk is warm (to the touch), add it to the yeast/sugar mixture, and stir until just combined. Let it rest for at least 10 minutes until yeast mixture starts to bubble
Add egg, egg yolk, sugar, vanilla extract, vanilla bean, cardamom, and salt to a large bowl, and mix well.
Add milk/yeast mixture to the to the egg mixture, and stir to combine.
Slowly add flour, stir to combine with a wooden spoon, then add softened butter. Once dough starts to come together, you can start kneading with your hands.
Cover with a warm damp dishtowel, and let it rise for at least 1 hour, or doubled in size.
Once dough has risen, dump it out on a clean surface, and divide into 12 equal pieces (use a scale for best results), and shape into buns, using a little flour if it's a little too sticky.
Let buns rest for 30-40 minutes covered with a warm, damp dishtowel.
After the buns have risen, you are now going to fill them with kagecreme. Flatten the buns into a disc. Add about a teaspoon of the kagecreme in the middle of the disc, make sure to not overfill, as it will explode when baking. Then form the disc back into a bun, making sure to pinch the sides together so the cream doesn't leak out of the bun.
Place the buns on a baking sheet lined with parchment paper. Cover the buns with a warm, damp dishtowel and let them rise another 30-45 minutes, or at least doubled in size.
Preheat oven to 350℉. Place a water filled deep (1-2") pan in the oven to act like a steam oven.
Once buns have risen, eggwash the buns, and bake for 12-15 minutes, or until golden brown.
Let the buns cool completed on a wire rack.