Add all ingredients, except butter, to the bowl of your stand mixer fitted with a dough hook. Mix until the dough comes together. Cover, and autolyse/rest for 30 minutes.
310 g bread flour, 20 g brown sugar, 7 g sea salt, 40 g whole milk, 55 g pure pumpkin purée, 70 g eggs, 130 g levain, all the tangzhong
Turn the mixer on again and knead at medium speed for 10 minutes. At this point, the dough should be sticky but slowly starting to release from the sides of the bowl.
Slowly add in cubed, softened butter. Continue kneading at medium speed for 15-20 minutes until your dough passes the window-pane test.
30 g unsalted butter
Place the dough on your work surface and roughly pre-shape the dough into a round using your hands and a bench knife.
Transfer the dough to a clean, lightly oiled bowl, cover, and place in a warm spot, ideally at 78°F/26°C. Proof for 3 hours.
After 3 hours, transfer the bowl with dough to the fridge for an overnight rest or 8-12 hours.