1. Make the tangzhong the night before baking: place the flour in a small saucepan and whisk in the water. Cook on medium heat stirring often, until it reaches 65C/149F – or significantly thickens but isn’t bubbling yet. Leave it to cool and cover the surface of the tangzhong with cling film. Refrigerate overnight.
2. The next day bring the tangzhong to room temperature. Mix the flour, sugar, salt in a large bowl or the bowl of your standing mixer. Add 120g of the tangzhong into the flour mix; discard the rest unless baking another loaf in the next couple of days. Crumble in the yeast and add the milk.
3. Knead with the dough hook attachment or by hand (it will be hellishly sticky) for 10-15 minutes until the dough becomes smoother and more elastic. Add the butter and continue kneading until the dough passes the windowpane test: when you pull up a little of the dough and stretch in your fingers, it should form a thin membrane and not tear.
4. Cover the bowl with the dough with cling film and leave in a warm place for 1 hour to double in volume.
5. Turn it out onto a lightly floured surface and divide in three equal pieces. Grease a large loaf tin with butter. Shape the dough pieces into balls, cover with a tea towel and let them rest for 15 minutes.
6. Roll each piece out into a long oval; fold it in three like an envelope. Flip it over, roll our again in the same direction; give it a quarter turn and roll up into a tight cylinder. Place it seam side down in the tin. Repeat with the other pieces of dough.
7. Cover the tin loosely with cling film and leave in a warm place to rise up to almost above the rim, about 30-40 minutes. Preheat the oven to 180C/350F/gas 4.
8. Brush the top of the risen loaf with the egg wash. Bake for 35 minutes; if the top of the loaf is getting too brown, cover it loosely with aluminium foil.
9. Remove from the oven and cool in the tin for 10 minutes. Turn the loaf out onto a wire rack and cool completely.