Written by 
Step 1
Combine the yeast, caster sugar, plain flour and salt in a bowl and whisk together thoroughly to combine.
Step 2
In a separate bowl, combine the warm water, rice bran oil and mashed potatoes and mix to briefly combine. In the bowl of a stand mixer fitted with a dough hook, add the wet ingredients to the dry ingredients and knead together until the dough is cohesive and slightly elastic, about 5-7 minutes. This is quite a wet dough, so try not to add too much extra flour.
Step 3
Set the dough aside to rise for 1-1½ hours or until it has doubled in size. Meanwhile, cut a sheet of baking paper into 10cm squares. (You will need about 20.)
Step 4
Knock back the dough and roll it out to 2-3cm thick. Cut out the doughnuts using a 7cm ring cutter and a 2cm ring cutter for the centre hole and set each doughnut onto an individual square of baking paper. This allows for much easier frying as the doughnuts won’t stick to the tray during the second rise.
Step 5
Allow the doughnuts to rise a second time for 30-45 minutes, or until they have doubled in size and puffed up.
Step 6
Heat the neutral oil to 180C in a large, deep pot.
Step 7
Place a doughnut (still on its baking paper square) into the oil – this will ensure you do not knock the air out of it while getting it into the oil (the baking paper will slip off during frying). I like to fry a couple at a time – if you overcrowd the pan, the oil temperature can drop and the doughnuts may become greasy. Fry until both sides are golden brown. Once cooked, they will float immediately. Scoop them out of the oil with a spider strainer and set aside on a baking rack set over paper towels to soak up any excess oil.
Step 8
Combine the sugar with the cinnamon and roll the still-warm doughnuts into the sugar mix before serving.