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.
Let the dough bulk ferment until it is jiggly and no longer sticky to the touch.
Stretch the dough out for lamination and add the blackberries and white chocolate chips. Fold the dough up, shape, and place in a floured banneton. Be gentle while shaping so the blackberries don’t get completely crushed.
Cold proof until ready to bake.
Preheat oven to 450°F. Score the loaf and bake covered for 30 minutes.
Remove the lid and bake uncovered for another 10–15 minutes. Keep an eye on the loaf toward the end to make sure the white chocolate doesn’t burn.
Cool for at least 1 hour before slicing.
