Preheat your oven to 220°C / 425°F. Spray or grease the donut pan. In a medium bowl, combine the sifted cake flour, sugar, baking powder and lemon zest and stir to combine.
In a separate bowl or jug, whisk the melted butter, eggs and buttermilk.
In a small bowl, mix one tablespoon of flour with the blueberries - make sure to coat the blueberries nicely.
Combine the flour mixture with the wet ingredients and the blueberries and stir until just combined. And make sure there are no visible lumps of flour.
Place a piping bag over a tall glass and spoon the donut batter into the bag.
Carefully pipe the batter into the pre-oiled pan. Fill the moulds until just under full. Do not fill to the top as they will rise quite a lot. Bake for 5-7 minutes until golden brown, and then let them cool on a wire rack.
Combine the powdered sugar, milk and vanilla and mix until you form a glaze.
To ice, the donuts, dip into the glaze and allow the glaze to set before serving.