Preheat your oven to 220°C / 425°F. Spray or grease the doughnut pan. In a medium bowl, combine the sifted cake flour, sugar, baking powder and stir to combine.
In a separate bowl or jug, whisk the melted butter, eggs and buttermilk.
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 doughnut 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, lemon zest and vanilla and mix until you form a glaze.
To ice the doughnuts, dip into the glaze and allow the glaze to set before serving.
Notes
Invest in good quality non-stick doughnut pans for easy release and perfect shapes.
Use frozen blueberries straight from the freezer to prevent colour bleeding into the batter.
Use a piping or zip-top bag with the corner snipped off for neat, even filling of the doughnut cavities.
Fill cavities only ⅔ full to allow room for rising. If you accidentally overfill, use a small spoon to remove excess batter before baking.
Mix the batter just until combined. Over-mixing can lead to tough, dense doughnuts.
Allow doughnuts to cool completely before glazing to prevent the glaze from melting off.