Preheat the oven to 180°C / 200°C fan.
In a bowl, beat together the caster sugar and butter until light and pale.
Add the egg and milk and beat until smooth.
Sieve in the flour and fold into the mixture.
Pour in the blueberries, and fold until they are evenly distrubuted throughout the mixture.
Spoon the mixture into the doughnuts mould, place in the oven for 13-15 minutes.
While cooking, mix the lemon juice and icing sugar until it forms a thick glaze, you can adjust this by adding more icing sugar.
Once baked, allow to cool in the mould before removing and place on a wired rack.
Spoon over the lemon glaze before serving.