Preheat deep fryer to 180C.
Place potato, flour, baking powder and baking soda in the bowl of a stand mixer fitted with the paddle attachment and mix thoroughly.
Whisk together oil, sugar, eggs and yoghurt in a small bowl.
Mixing on low speed, gradually add the wet ingredients and mix to form a dough.
Turn out the dough onto a floured bench and knead briefly to form a smooth dough.
Roll the dough to a 3cm thickness with a rolling pin.
Use a 7cm and a 3cm cutter to cut doughnuts.
Cook the doughnuts in batches in a deep fryer, including the doughnut holes, until golden.
Meanwhile, mix together the caster sugar and fennel seed.
Drain doughnuts on paper towel, then toss through fennel sugar.
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