Put the yeast, sugar and warm liquid (milk and water) into a bowl. Stir until the yeast fully dissolves, add the egg white and mix until smooth.
Add the flour, vanillin and salt, knead the dough, and at the end add the melted butter. Gather the dough into a ball and set it in a warm place for 1.5-2 hours.
While the dough is rising, make the filling: peel the apples and cut into small cubes. Melt the butter in a skillet, add the apples, sprinkle with sugar and cinnamon. Cook over low heat for about 10 minutes so the filling softens but doesn't turn to puree. Turn off the heat and let it cool.
Divide the risen dough into 8 pieces, round each one and place on a parchment-lined baking sheet. Leave to rise for 30 minutes.
Make the streusel: mix the flour with the sugar, add the chilled butter and rub it in by hand into a fine crumb. Keep it in the fridge.
Preheat the oven to 180 C. Make a well in the middle of each bun with the bottom of a glass. Brush the buns with egg yolk, fill with apple filling, and sprinkle streusel on top.
Bake in the preheated oven for 25-30 minutes. Cool the finished buns on a wire rack. Enjoy!