DAY 1:

Feed Sourdough Starter

• Feed your sourdough starter 4–6 hours before mixing, so it's active, at least doubled in size and domed on top.

Mix The Dough

• In the bowl of a stand mixer fitted with the dough hook, combine the flour, sugar and salt. Add the milk, eggs and active starter. Mix on low speed for 5–7 minutes until you have a shaggy, sticky dough.
• Cover and rest for 10–15 minutes. This hydrates the flour and makes the butter easier to incorporate.
• With the mixer running on medium speed, add the softened butter a few cubes at a time, letting each addition disappear before adding more. Keep mixing for 10–15 minutes until the dough is smooth and elastic and passes the windowpane test (stretch a small piece thin enough to see light through without it tearing).

Bulk Fermentation and Cold Proof

• Transfer to a lightly greased bowl, cover, and bulk ferment at 20–24°C (68–75°F) for 4–6 hours, undisturbed. Enriched dough won't double here, look for a gentle 20–25% rise and a slightly domed, puffy surface.
• Cover tightly and refrigerate for 8–16 hours. The cold proof firms the butter and makes the dough far easier to roll out the next day.

DAY 2:

Make The Filling

• Make the filling: warm the butter and cream in a small saucepan until the butter melts (don't let it boil). Off the heat, add the chopped milk chocolate, leave for a minute to soften, then whisk until smooth. Whisk in the cocoa powder and salt. Cool for 30–45 minutes until thick enough to hold its shape on the back of a spoon, milk chocolate sets softer than dark, so give it a proper cool before spreading.

Shaping and Second Rise

• Grease a 23x23cm (9-inch) square pan and line with parchment, leaving an overhang on two sides to lift the rolls out later.
• Turn the chilled dough onto a floured surface and roll into a rectangle roughly 40x30cm.
• Spread the cooled chocolate filling to the edges. Scatter over the crushed graham crackers and mini marshmallows.
• Cut the rolled-out dough into strips and then roll each of them up tightly, adding any marshmallows that try to escape. Alternatively, roll the dough into a tight log from the long edge and pinch the seam shut first.
• Trim the uneven ends, then cut into 9 even rolls (about 4cm thick) using a sharp knife or a length of unflavoured dental floss.
• Arrange the rolls cut-side up in the pan in a 3x3 grid, with a little gap between each, they'll expand and meet as they proof.
• Cover loosely with oiled cling film and proof at 22–25°C (72–77°F) for 4–5 hours, until visibly puffy, expanded by around 75%, and the rolls are touching.

Baking

• Near the end of proofing, preheat the oven to 180°C fan/200°C conventional (400°F).
• Bake for 28-30 minutes until golden. The marshmallow in the filling caramelises fast, so if the tops are colouring before the centre's cooked through, tent loosely with foil for the last 10 minutes.
• Cool in the pan for 25-30 minutes. You want warm, not hot, or the fluff topping will slide straight off.

Marshmallow Topping

• Spread the marshmallow fluff generously over the warm rolls.
• For a toasted finish, run a cook's blow torch lightly over the fluff, or slide the pan under a hot grill for 30–60 seconds. Watch it constantly, it goes from toasted to burnt in seconds.
• Scatter over the chocolate shavings (or chips), crushed graham crackers and extra marshmallows, and serve warm.