Apple Custard Crumble Buns with Cinnamon Streusel

Name: Apple Custard Crumble Buns with Cinnamon Streusel
Prep time: 30 min
Cook time: 25 min
Total time: 55 min
Cuisine: European, German
Ingredients: 4½ cups all-purpose flour; ½ cup whole milk (room temperature); ½ cup granulated sugar; 10 tablespoons unsalted butter (softened); 3 large eggs + 1 egg for egg wash; 1 packet instant yeast (2¼ tsp); Zest of 1 orange; Pinch of salt; 4 egg yolks; 1¼ cups whole milk; ¾ cup granulated sugar; ¼ cup cornstarch; ½ vanilla bean (split and scraped); 2 tablespoons melted butter; ½ cup all-purpose flour; 2 teaspoons granulated sugar; 2 teaspoons ground cinnamon; 2 apples (peeled and thinly sliced (or pitted cherries for the cherry variation)); Powdered sugar (for dusting)