How to Make It
In a large nonreactive saucepan, combine the rhubarb, sugar, cinnamon stick and vanilla bean and refrigerate overnight.
The next day, simmer the rhubarb mixture over moderately low heat until tender and the juices are slightly reduced, about 5 minutes. Let cool, then remove the vanilla bean and cinnamon stick.
In the bowl of a standing electric mixer fitted with the paddle, beat the butter with the confectioners’ sugar and granulated sugar at medium speed until smooth. Add the egg and egg yolk and beat until fully incorporated. Add the drained quark and beat at medium-high speed until smooth and fluffy. Add the bread crumbs and beat at low speed until thoroughly combined. Scrape the mixture into a medium bowl, cover with plastic wrap and refrigerate for 1 hour.
Line a baking sheet with plastic wrap. Fill a small plastic bag with the poppy seed filling and snip one of the corners. Using an ice cream scoop, scoop the dumpling mixture into twelve 2-inch balls. Working with lightly moistened hands, roll the balls until smooth. Make an indentation in the center of each ball with your finger. Pipe a scant 1/2 teaspoon of the poppy seed filling into each indentation. Pinch the dumplings closed and roll until smooth, rewetting your hands as necessary.
Bring a large pot of salted water to a simmer. Carefully drop the dumplings into the water and simmer over low heat just until they rise to the surface and are firm, about 12 minutes. Using a slotted spoon, transfer the dumplings to plates and serve with the rhubarb compote.
Quark, a tangy fresh cheese, is available at well-stocked specialty food stores. Poppy seed filling is available from nuts.com.