1. To make the starter, stir the milk and the cream into the flour in a small saucepan. Bring it to a simmer and cook over low heat for about 5 minutes, stirring continuously to eliminate any lumps, until it thickens and starts to come away from the sides of the pan as you stir. Scrape it into a small bowl, cover with cling film touching directly the surface of the tangzhong and leave to cool to room temperature.
2. In the meantime place the flour, sugar and salt in a large bowl or the bowl of a standing mixer with a dough hook attachment. Crumble in the yeast and mix it into the flour. Add the starter when cool, pour in the milk and mix until combined. Kneading by hand it will be tricky – the dough is very sticky – but if you apply the ‘grab underneath and fold dough onto itself’ technique, you might eventually get there. The standing mixer will do the trick in about 5 minutes, hand kneading will be considerably longer.
3. When the dough is smooth and elastic, and stops clinging to the sides of the bowl or your hands, add the butter, a small chunk at a time, and knead until it’s all incorporated. The final dough should be smooth, glossy and very stretchy. Place it in a buttered bowl, cover with a tea towel and leave to prove in a warm place for about an hour.
4. Butter a 12-hole muffin tin and preheat the oven to 180C/ 350F/gas 4. Turn the dough out onto a lightly floured work surface and divide it into 12 pieces (about 70g each). Shape each piece into a ball and drop it into the muffin tin hole. Place the tray in an inflated plastic bag (just blow into it and tie the end) and leave for 30 minutes to rise above the tray rim.
5. If you want to make just plain round rolls, shape the dough into 12 balls, place them on parchment lined baking trays and leave to prove as above.
6. Brush the tops of the rolls with double cream, rolls might want to be sprinkled with sesame seeds, and bake for 20-25 minutes until lightly coloured. Remove the rolls from the tin and cool on a wire rack.