Prepare the levain and fruit soaker (Day One, 9:00 p.m.) Mix the levain ingredients in a container and leave covered to ripen at about 78°F (25°C) for 12 hours overnight. In a separate bowl, cover the raisins and currants with water and let soak overnight.

Make the tangzhong (Day Two, 8:00 a.m.) Cook flour and milk portion in a saucepan over medium heat, whisking continuously, until the mixture thickens into a paste, about 5–8 minutes. Set aside to cool.

Mix (8:30 a.m.) Cut the butter into ½-inch pats and set on a plate to come to room temperature. To the bowl of a stand mixer fitted with the dough hook, add the flour, milk, egg, sugar, spices and zests, water, salt, tangzhong, and ripe levain. Mix on low speed for 1 to 2 minutes until combined. Increase to medium and mix for 5 minutes until the dough tightens and clings to the hook. Rest 10 minutes. Then mix on medium for 3 to 6 minutes, until smooth and strong. Add the butter one pat at a time on low speed, waiting for each to absorb. Add the inclusions (raisins and peel) and mix on medium for 1 to 2 more minutes until silky and elastic. Transfer to a bulk fermentation container.

Bulk fermentation (9:00 a.m. to 1:00 p.m.) Give the dough 2 sets of stretch and folds at 30-minute intervals. After the last fold, let the dough rest undisturbed for the remainder of bulk fermentation, about 3 hours.

Divide and shape (1:00 p.m.) Liberally butter a 9-inch square baking pan. Divide the dough into 9 pieces of 115g each and shape each into a tight round. Place in the pan in 3 rows of 3. Cover with a reusable plastic bag.

Proof (1:30 p.m. to 5:30 p.m.) Let the buns proof in a warm place for about 3 hours, or until very soft to the touch, well risen, and the dough has risen to the rim of the pan. Don’t rush the dough, keep it warm and proof until the dough is very soft.

Prepare the glaze, crosses, and bake (5:30 p.m.) Preheat the oven to 400°F (200°C). Make the simple syrup by simmering 50g water and 50g sugar until clear, 2 to 4 minutes; set aside. Mix the cross ingredients into a thick paste and transfer to a piping bag. Whisk the egg wash and brush lightly on the buns. Pipe crosses on each bun. Bake for 15 minutes, rotate the pan, reduce the oven to 350°F (175°C), and bake for 10 to 15 minutes more until the internal temperature reaches 195°F (90°C). Brush with simple syrup and let cool for 5 minutes before serving with salted butter.