Day 1: In a stand mixer on low speed, mix flour, water, milk, sourdough starter, sugar, salt, and optional yeast for 1 minute; add softened butter and chocolate chips and mix for 3?4 minutes until a smooth dough forms.

Let the dough rise in a covered bowl at room temperature until increased by 1.5 times (~3?4 hours), then refrigerate for 4?12 hours.

Meanwhile, prepare the butter block by forming a 8"?8" square of 340 g butter between parchment paper and chill until firm.

Day 2: Laminate the dough?enclose the butter block, perform a book fold, chill, then a single fold, and chill again. Let it rest overnight.

Day 3: Roll the dough into a ~20"?12" slab, cut strips (1"?6"), stack two strips with an offset, roll into a spiral, and place seam-side down in a muffin tin, poking a hole in the center.

Proof at room temperature for 4?6 hours until jiggly and doubled (or 2?3 hours in a proof box with steam).

Preheat the oven to 375?F; top each cruffin with a few mini chocolate chips, bake for 15 minutes, rotate, and bake for an additional 10?15 minutes until deep golden.

Cool completely before removing from tins and serving.