Mix the sourdough starter, water and flour together in a small bowl until fully combined.

Cover and leave to rise at room temperature for 8-9 hours, or overnight, until doubled in size. This makes about 120g of levain; you'll only need 100g for the dough (the extra allows for what sticks to the bowl, jar and spoon).

In a large mixing bowl or the bowl of a stand mixer, combine the strong flour, levain, sugar, eggs, milk, salt, vanilla paste and lemon zest. Bring together into a rough ball; you don't need to develop the gluten yet, just get everything combined. Knead by hand (expect a sticky dough) or use a stand mixer with a dough hook.

Cut the room-temperature butter into small cubes. With the mixer running, add the butter one cube at a time, waiting for each piece to incorporate before adding the next. Once all the butter is in, continue kneading until the dough is soft, elastic and glossy, about 10-15 minutes in total. By hand, knead the butter in the same gradual way; the dough will turn very sticky before it comes together, then keep slapping and folding it on the worktop for about 15 minutes until it's soft, elastic and noticeably less sticky.

Grease a large bowl with butter or oil and place the dough inside. Cover with a tea towel or cling film and leave to bulk ferment in a warm spot until doubled in size, roughly 3-6 hours depending on the temperature of your kitchen. Go by the rise, not the clock.

Once doubled, cover tightly and place the bowl in the fridge overnight.

Line your loaf tin with baking parchment (see equipment note above for the ideal size).

Take the dough out of the fridge and turn it out onto a lightly floured work surface. It should feel much firmer and far less sticky than the day before. Roll it into a log to knock out the air.

Divide the dough into 10 equal pieces. Shape each piece into a tight, smooth ball by pulling the edges out and folding them into the centre.

Arrange the balls snugly in the tin. Cover with a tea towel or lightly greased cling film and leave to proof again in a warm spot, ideally 21-23°C, for 4-6 hours, until just shy of doubled. If your kitchen is cool, turn the oven on its lowest setting for 2 minutes, then turn it off and place the tin inside to proof. Don't let the dough overproof, or the loaf may collapse in the oven.

Preheat the oven to 200°C fan. While it heats, lightly whisk the egg yolk and brush it generously over the top of the loaf.

Bake in the centre of the oven for 25-30 minutes, until deep golden brown. Cool in the tin for at least 15 minutes before pulling it apart. For clean slices, cool completely before cutting.