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Proofing your sourdough doughnut dough allows for the dough to become fluff and airy. The first proof, before stretching and folding, should be done at a, what I call “comfortably chilly”, temperature. Not as cold as your fridge, but certainly not as warm as your living room. About 15°C/59°F.
If your dough proofs too fast, your doughnuts will burst when you fry them next day. It’s better to proof longer and slower (at lower temperature), than too quick.
After stretching and folding, the dough needs to be proofed overnight in the fridge.