Work the flour, quark, eggs, milk and salt into a smooth dough. If necessary, add a little more flour, the spaetzle dough must be tough and difficult to tear from the spoon. Let the dough rest for about an hour and stir every now and then.
In the meantime, prepare the compote. To do this, wash the plums, stone them and cut them into pieces. Bring to the boil together with the apple juice and the cinnamon stick. Then take the cinnamon stick out again. Mix the custard powder with the vanilla sugar and some juice or water and stir into the compote. Let simmer until the compote thickens a little.
Now put the dough into boiling salted water through a spaetzle press or grater and bring to the boil once. Scoop the spaetzle out of the pot when it floats and drains off.
Melt the butter in a pan and toss the spaetzle in it. Add poppy seeds and powdered sugar and stir in. Serve with the plum compote.