Add the flour, yeast, sugar and salt to a large mixing bowl. Mix well and make a well in the centre.
Pour in the lukewarm oat milk. Mix until the mixture forms a smooth dough. Cover with a tea towel and leave to double in size, approximately 1.5 hours.
Gently fold the dough, knocking out the air. Portion out the dough into 8 large portions, or if preferred, 16 smaller portions.
Over a high heat, add enough oil in a large saucepan to allow for deep frying. This should be roughly a litre of oil.
Test the heat of the oil by adding a drop of batter; it is the correct temperature if the batter sizzles and floats to the surface.
Using 2 wooden spoons, drop the portions of dough into the hot oil. Use a ladle to roughly shape the dough into balls.
Cook for 3-3.5 minutes on one side until a deep golden-brown. Flip the doughnuts over and cook for a further 2-2.5 minutes. Remove and place on a plate lined with kitchen paper. Sprinkle over caster sugar, then leave to cool
Meanwhile, in a saucepan heat the cream to a light simmer or alternatively place in the microwave for 30 seconds. Remove from the heat and whisk in the chocolate until the sauce is smooth and glossy.
Whisk in the butter until fully melted. If necessary, heat the sauce for 10 seconds at at time. Stir in the sieved cornstarch.
Place in the fridge to cool for 30 minutes. Remove and fill a piping bag to two thirds full with the chocolate cream.
Cut a small slit on the side of each doughnut. Pipe in the chocolate until it begins to emerge from the opening. Serve and eat immediately!