In a large mixing bowl, combine all-purpose flour, warmed whole milk, a large egg, instant yeast, granulated sugar, vanilla sugar, and salt. Knead the mixture for 5-10 minutes until the dough becomes smooth and elastic. Cover and let rise for 1 hour.

Peel, core, and dice the apples. In a skillet, melt butter over medium heat, add the apples, brown sugar, and ground cinnamon. Sauté for 5-7 minutes until soft. Stir in cornstarch mixed with water, cooking until thickened. Cool completely.

In a medium bowl, whisk together cream cheese, granulated sugar, and vanilla sugar until creamy and smooth. Set aside.

Once the dough has risen, punch it down and divide into 9 equal portions. Roll into smooth balls and let rest for 15 minutes.

Use a floured glass to press down in the center of each dough ball, creating deep wells.

Brush edges of the indented dough with beaten egg yolk. Carefully dollop 1 tablespoon of cream cheese mixture followed by 1 tablespoon of cooled apple filling into each well.

In a bowl, combine cold butter, flour, and sugar to create a crumbly mixture. Sprinkle this topping over each filled bun.

Preheat oven to 180°C (350°F). Arrange buns on a parchment-lined baking sheet and bake for 20-25 minutes until golden brown.

Prepare glaze by whisking powdered sugar with a splash of milk until smooth. Drizzle over the cooled buns.