Mix the starter and water, then add the bread flour and salt. Mix until combined and rest for 1 hour.
Do 4 sets of stretch and folds, 30 minutes apart. During the second set, gently stretch the dough out and add half of the frozen grated butter before folding. During the third set, add the remaining frozen grated butter and fold it into the dough.
Let the dough bulk ferment until it is jiggly and no longer sticky to the touch. Shape the dough and place it in a floured banneton.
Cold proof until ready to bake.
Preheat oven to 450°F. Score the loaf and bake covered for 25 minutes. Place a cookie sheet on the rack beneath to prevent the bottom from getting too dark.
Remove the lid and bake uncovered for another 20 minutes. Keep an eye on the loaf toward the end to prevent it from getting too dark.
Cool for at least 1 hour before slicing.
