First, make the tangzhong.To do this, place the flour, milk & water into a small saucepan & whisk to combine. Cook over a low heat until you have a thick paste, stirring constantly with a spatula. Transfer to a bowl & leave to cool to room temperature. Next, place the milk & water into a jug & warm in the microwave to 38°c/100°f (alternatively you could heat this up in a saucepan then transfer to a jug), then whisk in the yeast & leave to go bubbly, 5-10 minutes.
To the bowl of a stand mixer, add the flour, salt, sugar, milk powder, egg, oil & the cooled down tangzhong. Give the activated yeast a whisk then add to the bowl. Using the dough hook, knead the dough on a medium speed until smooth & starting to come away from the sides of the bowl, around 5 minutes.
With the mixer still going, add in the butter a small amount at a time, incorporating each piece fully before adding more. Once all the butter has been incorporated, keep mixing until the dough passes the window pane test, 5-10 minutes.
Transfer the dough to a lightly oiled bowl, cover with clingfilm or a tea towel & leave to double in size at room temperature, 1-1.5 hours.
Grease a 8" x 12" baking tin with butter or oil.
Once risen, knock the dough back then transfer to a lightly floured work surface. Using a metal bench knife, portion the dough into 12 equal pieces. They will weigh around 60g each.
Take a piece of dough & flatten slightly. Place a piece of mozzarella into the centre then fold the sides over. Flip the dough upside down & use your hands to shape into a ball. Place seam side down into the baking tin.
Repeat with the remaining dough. In your baking tin, you should have four rows of three dough balls. Make sure that they are spaced equally apart.
Cover the tin loosely with clingfilm & leave to rise at room temperature for 45 minutes.
In the meantime, preheat an oven to 180°c/160°c fan (350°f/320°f).
Once the dough is well risen, whisk together the egg & water for the egg wash. Brush this all over the top of the dough then sprinkle over the sesame seeds.
Bake in the preheated oven for 20-25 minutes or until a deep golden brown.
Mix together the melted butter, garlic, salt & parsley then brush a generous amount over the bread, whilst it's still hot.Leave to sit for a couple of minutes then grate over the parmesan.Allow to cool slightly then serve.To remove the bread from the tin, run a palette knife around the outside & in between each bun. You will then be able to pull the bread out with your hands.