Roux

1. Combine the water and flour in a small saucepan and whisk on medium heat until thickened into a paste
2. Remove from the heat, cover, and cool until room temperature

Dough

1. Combine bread flour, all-purpose flour, milk powder, sugar, salt, and yeast in the bowl of a stand mixer fitted with a dough hook
2. Add the egg, water, and cooled roux and mix on medium for 3 minutes or until a rough dough forms
3. Add the butter and continue to mix for 15-20 minutes until the dough is smooth and elastic
4. Roll the dough into a ball and place it in an oiled large bowl
5. Cover loosely with cling wrap and place in a warm place for 1-2 hours or until doubled in size
6. Transfer the proofed dough to a floured work surface and press out the excess air
7. Divide the dough into 10 equal portions and roll each into a ball
8. Place them on a large baking sheet lined with parchment paper
9. Cover with cling wrap and place in a warm place to proof for 30 minutes, or until almost doubled in size
10. Meanwhile, preheat the oven to 180C/355F
11. Combine the egg yolk and milk, and whisk until well combined
12. Uncover the proofed buns and brush with the egg wash
13. Bake for 13-15 minutes, or until golden brown
14. Transfer the buns to a wire rack and allow to cool completely

Assembly

1. Combine the cream, sugar, and vanilla in the bowl of a stand mixer fitted with a whisk attachment or a large bowl with an electric mixer and whisk until it reaches stiff peaks
2. Make a slit in each bun, making sure you don't cut all the way through
3. Fill the slit with cream and clean up the edges of the bun
4. Add fruits if desired, dust with powdered sugar, and enjoy!