Mix the dough: In a large bowl, combine sourdough starter, warm water, and bread flour. Mix until there are no dry spots--it will resemble a shaggy ball. Cover and let rest for 30–45 minutes (autolyse).
Add salt: Sprinkle the salt over the dough and mix until incorporated.
Bulk fermentation: Cover and let the dough rise at room temperature for 3–4 hours, performing 3–4 sets of stretch and folds every 30 minutes.
Add inclusions: During the second set of stretch and folds, gently fold in the rainbow sprinkles and white chocolate chunks. Be careful not to overmix so the sprinkles don’t bleed.
Pre-shape: Once bulk fermentation is finished, lightly flour your work surface.
Gently turn the dough out of the bowl, being careful not to deflate it.
Using a bench scraper, tuck the edges of the dough toward the center to form a loose round.
Flip the dough seam-side down and use your bench scraper to gently pull the dough toward you, tightening the surface to create tension.
Let the dough rest, uncovered, for 20–30 minutes. This resting period helps relax the gluten, making the final shape easier.
Final shape: Lightly flour the top of the dough, then flip it over so the floured side is down.
For a round (boule): Fold the bottom third of the dough up, then fold the sides in like an envelope. Finally, roll the top down toward you, creating surface tension as you go.
For an oval (batard): Fold the bottom up, then fold each side inward. Roll the dough down from the top, lengthening it slightly as you tuck it into an oval shape.
Place the shaped dough seam-side up into a floured banneton or a bowl lined with a floured kitchen towel.
Cover and refrigerate overnight (8–12 hours) for the final proof.
The next day, preheat the oven: Place a Dutch oven inside your oven and preheat to 450°F (232°C). Let the Dutch oven preheat for 30-60 minutes.
Score and bake: Turn the dough onto parchment paper*, score the top, and carefully transfer it into the hot Dutch oven. Cover and bake for 30 minutes. Remove the lid and bake for an additional 25 minutes, until golden brown.*Tip: To prevent the bottom from getting burnt, add some uncooked rice to the bottom of the Dutch oven, add the parchment paper, and a bread sling if you have one, before adding the dough to the pan. Cool: Transfer the bread to a wire rack and let it cool completely before slicing.