In a large bowl, add flour, baking powder, salt, & sugar. Mix dry ingredients.
Add in egg, milk, vanilla extract, lemon zest, and lemon juice. Mix until combined.
Add in oil and stir.
Place pastry bag in a pint glass and fold over top. Add donut batter into the bag, set aside.
Using cooking spray or silicone brush, oil donut pan wells.
Cut tip off disposable pastry bag and fill donut pan wells 3/4 full.
Place donut pan on middle rack in oven.
Bake for 9-10 minutes.
Let cool for 1 minute, then remove from pan and let cool on wire rack.
Once doughnut cools completely, top with glaze.
To Make Glaze
Combine powdered sugar and lemon juice until desired consistency. Dip top half of donut in glaze and set on cooling rack to set. Top with additional lemon zest if desired.