Combine quark, cream cheese, cream, sugar, vanilla sugar, and cream stabilizer in a large bowl. Mix until smooth.
In a pan, heat butter and sugar over medium-high heat, stirring frequently, until the sugar melts and becomes golden (about 3–5 minutes).
Add flaked almonds and stir constantly for 1–2 minutes until lightly browned.
Immediately spread the hot brittle onto aluminum foil or a baking sheet. Let cool completely for about 10 minutes, then break into smaller pieces if desired.
In a glass bowl, layer the quark mixture with blackberries, then sprinkle the brittle pieces on top.
Refrigerate for at least 2–3 hours before serving.