makes 8–12 rolls depending on size; adapted from Maurizio Leo, The Perfect Loaf
Mix starter, flour, and water in a pint sized container. Cover loosely and let sit at room temperature overnight.
Cut the butter into ½-inch pats and set on a plate to come to room temperature.
Soak currants and/or raisins in a small bowl of warm water and set aside.
Combine the flour, levain, milk, water, sugar, and egg in the bowl of a stand mixer with the dough hook attachment. Once combined, mix on medium speed with the dough hook for 5 minutes then let dough rest, covered, for 10 minutes.
Add salt, spices, citrus zest (optional), and mix on low for 2 minutes. Turn the mixer up to medium speed and mix for 6 minutes to develop the dough—it should pass a "windowpane test" and if not, mix a couple minutes more.
With mixer running on medium speed, add the butter, one pat at a time, mixing until incorporated before adding the next pat. Once all the butter has been added, mix on medium speed for 2 minutes and add the add-ins (well-drained currants/raisins, candied citron, candied orange). The dough should be smooth, elastic, and shiny.
Begin bulk fermentation with 3 sets of stretch-and-folds at 30-minute-intervals, either in the bowl or by removing the dough from the bowl and doing it on the counter.
How to do a set of stretch and folds in the bowl: Wet your hands and starting at the "noon" position of your bowl, scoop the dough from the bottom of the bowl up and fold it over towards the middle of the dough ball. Repeat this every quarter turn around the bowl—from "noon" to "3 o'clock" bottom stretch to middle, "6 o'clock", "9 o'clock"—one complete trip around the clock equals one set of stretch and folds.
Let dough rest for the remainder of bulk fermentation; at the end the dough should be risen, jiggly, and "alive" feeling. If not, leave for 15 more minutes and check again. As Maurizio says, "The dough should be lofty at this point; don't rush it."
Butter baking pan—an 8-by-8-inch square for 9 buns or a larger rectangle for either 8 or 12
Using a bench scraper, divide the dough into desired number of buns then round each into a taught ball, stretching the dough from the top, down the sides, and tucking it underneath. Transfer to buttered pan.
When all buns are in the pan, cover loosely with a piece of oiled-plastic wrap and set in a warm place to proof for 3–4 hours.
Alternatively, place covered dish in the refrigerator for a cold proof. In morning, remove dish from refrigerator and allow to proof until buns are puffy, 3–4 hours.
Preheat oven to 400° F. Prepare egg wash and brush over the tops of the buns.
Mix the cross paste with flour, water, and a pinch of salt in a small bowl until smooth—you want the paste thick enough to pipe and hold its shape.
Transfer the paste to a piping bag with a small tip or use a plastic sandwich bag with one corner nipped off. Pipe across the buns.
Bake for 15 minutes at 400° then lower heat to 350°, rotate pan 180°, and continue to bake until finished, 10–15 more minutes until buns are golden brown with internal temperature at least 195°. Optionally brush tops of buns with melted jam or honey (see note.)