• Mix all the starter ingredients until well combined. Scoop the mixture into a clean jar covered loosely and leave it to double.
• Once doubled, add flour, sugar, and salt in a large bowl. Add water, milk and sourdough starter and mix into a shaggy dough. Tip this dough onto a workbench and use a slap-and-fold kneading method to bring the dough together. Knead it for 5 minutes to begin developing the gluten. Fat can coat the flour particles, inhibiting gluten formation, so I like to give the gluten a head start before adding the butter.
• Add in the cubed butter, a few cubes at a time, and knead them in. The butter will make it sticky, but the dough will already have some strength from the previous kneading.
• Continue slapping and folding for a further 3-4 minutes until smooth (but still a bit sticky.) This can also be done with stand mixer and dough hook.
• Overnight fridge proof: Add the dough to a lightly greased bowl, cover it and let it rise until it has almost doubled in a warm spot (ideally around 25°C/77°F) for 3-4 hours. You can create a warm and humid spot by placing the dough into a turned-off oven alongside a large cup of boiled water. Replace the water as needed when it cools down. After this period, tightly cover the dough so it doesn't dry out and refrigerate overnight.
• No fridge proof: If you’re skipping the fridge proof, make the dough and let it double in size at room temperature, then move onto the shaping steps.

Shaping

• Punch down the dough and pull onto a floured work surface. Shape it into a dough ball and weigh it on a scale. Divide the total weight of your dough by 9. That's how much dough is needed for each bun.
• Shape each piece into a tight ball. I use a claw-hand technique: Curve your fingers and place your hand over the dough. Gently press and rotate the dough in small circles on the surface. Keep rotating until the dough forms a smooth, tight ball with a taut surface.
• Place the buns on a baking tray lined with parchment paper. Dip your fingers in water and use them to gently flatten the dough ball slightly. This will help to make a wider bun.
• Loosely cover and leave the buns to rise for approximately 2-4 hours in a warm spot until they have about doubled in size.
• Preheat the oven to 200°C/400°F.
• Lightly beat an egg with a tablespoon of water. Brush this over the buns and sprinkle them with sesame seeds.
• Bake for approximately 22-25 minutes until the buns are a deep golden brown. Let them cool on a wire rack.