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, mix for 3?4 minutes until smooth dough forms.

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

Prepare the butter block by forming an 8"?8" square of 340 g butter between parchment and chilling until firm.

Laminate the dough by enclosing the butter block, performing a book fold, chilling, then a single fold, and chilling again; rest overnight.

Roll the dough into a ~20"?12" slab, cut into strips measuring 1"?6", stack two strips with offset, roll into a spiral, and place seam-side down in a muffin tin; poke a hole.

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

Preheat 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.