makes 8–12 rolls depending on size you want; 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.
Combine the flour, levain, milk, water, half of the sugar (21 grams), egg, and salt 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 remaining sugar (21 grams), spices, citrus zest, 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. The dough should be smooth, elastic, and shiny.
Drain the currants and/or raisins and pat dry with a paper towel.
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. At the first stretch-and-fold (at 30 minutes), add the drained and dried currants (or other add-in) by sprinkling a quarter of the amount on top of the dough at each stretch and fold until all is combined. As the fermentation continues, the ingredients will incorporate well into the dough.
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. By the end of the time, 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 interior of baking pan, an 8-by-8-inch square or larger rectangle work well.
Scoop dough from bowl onto counter. Divide into equal pieces, anywhere from 8 to 12 rolls depending on your desired bun size and baking dish you want to use.
Using a bench scraper, round each piece into a taught ball, stretching the dough from the top, down the sides, and tucking it underneath itself. Work around the dough ball a few times to get it nice and tight. 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.
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 cross paste to be thick enough to pipe and hold its shape somewhat; start with equal parts of flour and water, adding more water to thin out the paste. Transfer the paste to a piping bag with a small tip or use a plastic sandwich bag and nip off a small piece at one corner. Pipe across the pan in three stripes then rotate 90° and pipe across three more stripes.
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 195°. Transfer pan to wire rack to cool completely. Optionally brush tops of buns with melted jam or honey (see note.)