Slow proofing simply is retarding, or slowing down the proofing process of the dough. The benefits of slow proofing is allows flexibility to make the donuts at a later time but more importantly, it improves the texture and flavour. Proofing the donut dough in the fridge helps the dough develop a deeper flavour, its also gives it better structure, firms up the butter and makes it much easier to work with. Cold dough is less sticky firmer than than a room temperature one.