In a medium bowl, combine the levain, water, eggs, and maple syrup. In a large bowl, combine the flour and salt. Add the wet mixture to the flour and mix with a stiff spatula or an Einkorn Kneading Tool until the flour is almost absorbed. Work the butter into the dough until it is completely absorbed. Cover the bowl with a plate and let stand for 15 minutes.
Lightly flour a work surface and using a bowl scraper, transfer the dough to it. Turn the dough by stretching the dough into a rectangle, then folding in each corner to the center. Fold again in half, then transfer the dough back to the bowl and cover tightly with the plate. Let the dough rest for 15 minutes, then turn the dough 2 more times at 15-minute intervals.
After the last turn, transfer the dough back to the bowl, but this time, cover the bowl tightly with plastic wrap. Let the dough rise for 3 to 5 hours. You should see your dough rise by 30%. If your sourdough starter is new, it might take longer. If it is very hot in your kitchen, the dough may rise quicker. After the dough has risen, if you do not have time to bake it right away, you can place the dough in the refrigerator. The cold temperatures will slow down the rise considerably.
To shape the loaf, you’ll stretch and tuck from the top of the piece of dough toward the center, working with one-quarter of the dough and pressing down to seal the seam as you go. Repeat this three times, then pinch the last seam to seal completely. Roll the loaf a few times on the counter to finish shaping it.
Place the dough, seam side down, in the prepared pan. Cover the pan with buttered plastic wrap. Let the dough rise at room temperature for 60 to 90 minutes, or until the top of the loaf rises about ½ inch below the rim of the pan. If your starter is new, this may take up to 2 hours.
Preheat the oven to 425°F.
Place the bread in the oven, then lower the temperature to 375°F. Bake the loaf for 40 minutes, until dark brown.
Place the pan on a wire rack to cool for 15 minutes, then turn out the loaf and let it cool completely for 2 hours before slicing. If you like a soft crust, place a clean towel over the loaf while it cools, so the steam is absorbed into the crust.
Store in a loosely sealed plastic bag for up to 3 days, or freeze for up to 1 month.