Heat the milk and water until they are just lukewarm.
In a large stand mixing bowl, add flour and sugar, and whisk together.
Make a well in the center and pour in the yeast and then the milk and water mixture.
Stir a little flour into the milk, water, and yeast mixture. Cover the bowl with a towel and set it somewhere warm to rest for 10 minutes.
After the 10 min have passed, give the bowl a quick stir with a rubber spatula.
Add the butter, egg, egg yolk, and vanilla extract.
Use the dough hook on low speed for 10 min or until the dough is a smooth ball and the sides of the bowl are clean.
Shape the dough into a ball and place in a greased bowl.
Cover the bowl with a towel/proofing bag and set it somewhere warm to rest for 30 minutes.
Shape the Hefeklösse
After the 30 minutes have passed, knead the dough briefly by hand to knock the air out.
Roll the dough out on a lightly floured surface until it is about 1/2" thick.
Use a 3" circle cutter to cut out as many disks as possible.
Place the disks on a lightly floured baking sheet to proof.
Knead the remaining dough and re roll to the same thickness. Cut out one more set of discs and place on the baking sheet to proof.
Cover the disks with a towel or proofing bag and leave to rest for 20-30 minutes.
Immediately set up the steam bath.
Steam the Hefeklösse
To cook these buns, you need a large pot - ideally it will have a domed lid but if not use another pot that is the same size or a bowl with the same diameter to place upside down over the pot.
Fill the pot with about 2" of water and place a thin dish towel over the top and secure with a rubber band or a piece of string so that the dish towel creates an almost taught surface across the top of the pot.
Cover the towel with the domed lid, same sized pot, or same diameter bowl and turn on the heat to medium/high to bring the water to a boil.
After the buns have doubled in size and the water is boiling, place a few of the hefeklösse on to the towel and cover with the lid and leave to steam for 10 minutes.
Once steamed, remove the first batch from the towel and place into serving bowls.
Repeat the steaming process with the remaining hefeklösse.
While the next batches steam, start eating the first batch. Tear them into 1/2" chunks and top with sour cherries and juice.