• In a large bowl, stir 360 g warm water with 30 g granulated sugar, 10 g vanilla extract, and 1 g butter extract. Stir in 100 g active sourdough starter until mixture is milky.
• Add 500 g bread flour, 10 g nonfat milk powder, and 10 g salt and mix until evenly hydrated with no dry spots remaining. Cover bowl and let rest for 30 minutes.
• After 30 minutes, stretch and fold the dough. With wet hands, lift one side of the dough, stretch it up, and fold it over itself. Rotate the bowl ¼ turn and repeat until you’ve done 4 folds total (a full 360 degree rotation). Cover and rest 30 minutes between sets.

  On the 3rd set: Before each of the four folds, scatter a portion of the frozen rainbow sprinkles over the exposed dough and then make the fold to trap them inside. Brush off any sprinkles sitting on the surface.
• After final stretch and fold, cover bowl and let rest for remaining bulk fermentation. This will take about 4-8 hours, until the dough has grown about 75% in size, is jiggly, bubbly at the edges, and has a slightly domed center. How long it takes will depend on many factors like how warm your kitchen is (will rise faster in warmer spot), how active your starter is (active starter will rise faster), etc.
• Once bulk fermentation is complete, turn dough out onto a lightly floured surface and gently work into a smooth ball. Let rest for about 10 minutes.
• While dough is resting, line a bowl with a kitchen towel or banneton and lightly dust with flour. Gently stretch and pat the dough ball into a soft rectangle taking care not to deflate it. Fold dough like a hot-dog roll (starting with the long edge closest to you, fold ⅓ of the dough towards the middle, and then the other top ⅓ of the dough towards the middle to overlap the first folded layer). Shape into a taut round, using a bench scraper to rotate and drag the dough toward you to create a tight skin. Transfer to prepared bowl, seam-side up.
• Cover bowl and place in the fridge overnight, for at least 8 and up to 18 hours. I like to remove the cover for the last 30 minutes in the fridge to dry the skin out a bit.
• When ready to bake, put dutch oven in oven and preheat to 475 degrees. While oven is preheating, invert cold dough onto a piece of flour dusted parchment paper, dust top of dough lightly with flour, and score with sharp knife.
• Place dough on parchment paper in hot dutch oven, cover, put in oven, and immediately turn oven temperature down to 450 degrees F. Bake 30 minutes. Remove cover, lower oven temperature to 400 degrees and bake for another 5-10 minutes until golden. Internal temperature of bread should be about 205-210 degrees. If color on top of bread darkens too quickly, you can tent that area with a thin strip of foil while bread is baking uncovered (or put the cover back on).
• Allow loaf to cool completely before adding icing (if using) and slicing.

Vanilla Icing

• Add 90 grams confectioners' sugar, 15 grams milk, 4 grams vanilla, and pinch of salt to a medium bowl and whisk until smooth.
• For a thinner icing, add additional milk.