Preheat oven to 325ยบ. Lightly grease two 6 count donut pans with cooking spray or vegetable oil.
In a medium bowl, whisk together flour, sugar, baking powder, baking soda, and salt. Set aside.
In a small bowl, whisk together egg, yogurt, oil, buttermilk and lemon zest. Add to dry ingredients and stir just until combined.
Divide batter evenly into 12 donut cups, filling each about half full. (Use a pastry bag or plastic bag with corner cut out to make filling the cups easier.)
Bake for 10-15 minutes, until a tester inserted into the center comes out clean. Remove from oven and cool for a few minutes in the pans.
Whisk together powdered sugar and lemon juice to create a thin glaze.
Remove donuts from pans and dip into the glaze. Sprinkle if desired with colored sprinkles. Cool on a wire rack.