Combine starter, water, salt, and flour in a bowl

Mix until a shaggy dough forms

Cover and rest for 30 minutes (hydration step)

Do first stretch & fold without butter

Before second set → add half the grated frozen butter

During third set → add remaining butter

Fold gently to incorporate

Let dough rise at room temp for 4–8 hours

It should:

roughly double

look bubbly and elastic

Shape into a loaf

Place in greased loaf pan

Cover and refrigerate for ~12 hours

Preheat oven to 450°F (232°C)

Bake:

25 min covered (lid or foil)

25 min uncovered

deep golden crust

internal temp ~200°F (93°C)