You'll love this simple rich, thick, and creamy dessert!
Heat milk to a simmer, then take off the heat and let cool to room temperature.
Whisk buttermilk into milk. Pour into a bowl or into glass or plastic jars. Cover with a clean tea towel and let sit on the counter for 12-36 hours.
When the Quark has thickened, pour into a clean tea towel (or muslin or cheesecloth). Gather the corners and use a rubber band to hang the Quark from a spoon placed on a large bowl. Let it sit in the fridge for several hours (or overnight).
Whip Quark and heavy cream until thick. Optional: a few drops of vanilla extract or a spoonful of vanilla sugar.
Add blackberry jam and whip until blended. Chill in the fridge until you're ready to serve. Garnish with fresh fruit.