Make the pre-ferment: Mix all pre-ferment ingredients in a small container and leave to rest at 25°C for 5 hours.
Autolyse: In a medium bowl, mix the wheat flour, wholegrain flour and 650 g water until no dry patches remain. Cover and rest for 2 hours.
Mix the main dough: Add 20 g water (less if the dough feels too wet), 17 g salt and the ripe pre-ferment to the bowl. Work in with your fingers, then stretch and fold for 2–3 minutes until the dough is smooth and cohesive. Transfer to a proofing box or large bowl and cover.
Bulk fermentation: Stretch and fold the dough 6 times in total — 3 times every 15 minutes, then 3 times every 30 minutes.
Divide and pre-shape: Lightly flour the work surface, turn out the dough and divide into two equal halves. Loosely pre-shape each piece into a round and leave to rest uncovered for 30 minutes.
Laminate: Gently stretch the dough into a rectangle and grate the ice-cold butter evenly over it. Fold the bottom third up, then fold the right side to the centre and the left side over it. Roll the dough up from the top, building tension as you go. Press the seam firmly to seal in the butter. Place seam-side up in a floured proofing basket.
Final proof: Cover the proofing baskets and refrigerate overnight for at least 12–14 hours.
Bake: Preheat the oven to 230°C. Place a cast iron pot (e.g. Le Creuset / Dutch oven) inside and heat for 30 minutes. Remove the dough from the fridge, place directly into the hot pot and score. Bake covered for 20 minutes, then remove the lid and bake for a further 30 minutes. The bread is ready when the internal temperature reaches 97°C.
Cool: Leave to cool on a wire rack for at least 2 hours before slicing.
