Preheat the oven to 175°C (350°F), top and bottom heat. Line a 6-cup muffin pan with paper liners.
Mix the melted butter, sugar, vanilla, eggs, and milk. Stir in the all-purpose flour, baking powder, and salt until just combined, then fold in the diced apple.
Fill the muffin cups about ⅔–¾ full.
Mix the quark or unsalted cream cheese, sugar, egg yolk, cornstarch, and vanilla until smooth. Spoon a generous dollop onto each muffin.
Top with the apple slices, sprinkle with the sliced almonds, and finish with the cinnamon-sugar mixture.
Bake at 175°C (350°F) for 25–30 minutes, until golden brown.
Let cool completely and lightly dust with powdered sugar before serving.
