• Commence by preheating the oven to 350°F. Simultaneously, prepare the donut pan by lightly greasing it.
• In a large bowl, meticulously sift the flour, baking powder, and salt together. Once done, set it aside.
• In a separate mixing bowl, blend together the buttermilk, granulated sugar, eggs, vegetable oil, previously melted butter, finely grated lemon zest, and vanilla extract.
• Gradually incorporate the sifted dry ingredients into the blended wet mixture, ensuring a smooth batter.
• Carefully fold the fresh blueberries into the batter.
• Transfer the batter to the previously prepared donut pan, ensuring even distribution.
• Place the pan in the preheated oven and allow the donuts to bake for approximately 17 to 20 minutes.
• Once baked, remove the donuts from the oven and let them cool for a duration of 10 minutes. Subsequently, transfer them to a wire rack for further cooling.

For the Glaze:

• In a suitable bowl, whisk together the powdered sugar and freshly squeezed lemon juice until a smooth consistency is achieved.
• Position a foil beneath the wire rack holding the donuts to capture any excess glaze. Generously pour the glaze over each donut, ensuring they are adequately coated.
• Conclude by garnishing the glazed donuts with the finely grated lemon zest.