Spray your donut pan maker with cooking spray and preheat it.: As the pan warms, you will notice a faint metallic scent and a little steam if it is electric, which indicates it is reaching temperature. Preheating ensures the batter begins to set immediately on contact, producing a tender edge and even rise, and spraying prevents sticking so the donuts release cleanly. A typical mistake is skipping the spray or preheat, which can cause donuts to cling and break when removed.
In a large mixing bowl, beat the egg, lemon juice, lemon zest, and the Splenda on high with an electric mixer until the mixture is light and foamy. Set aside.: As you beat, the mixture should transform into a pale, airy foam, and you may smell the lemon oils become more vibrant. This aeration traps tiny bubbles that lighten the final crumb, which is why vigorous beating matters at this stage. Avoid overbeating to a dry foam, as that can lead to collapse when folded with denser ingredients.
In a separate medium bowl, combine the whole-wheat pastry flour, baking powder, and poppy seeds.: When you whisk these dry ingredients, you are distributing the baking powder and the poppy seeds so the rise is consistent and seeds are evenly scattered. The flour will look slightly more buff than all purpose, and the little black flecks of poppy seeds will stand out visually. Do not skip this step, since uneven leavening can create tunnels or dense pockets.
Slowly stir the dry mixture and the sour cream into the egg mixture. Fold gently until well combined.: After adding a bit of the dry mix to the wet, the batter will look creamy and speckled with poppy seeds. The folding motion preserves air and prevents developing gluten, which keeps the donuts tender. If you stir too vigorously, you might end up with a gummy texture, so use gentle, sweeping folds until everything just disappears into a uniform batter.
Drop a tablespoonful of the mixture into each donut spot and close the lid.: As you fill the cavities, the batter should hold shape briefly before settling, and the poppy seeds will dot the surface. Filling from a spoon or piping bag helps control portions and keeps the donuts uniform. Overfilling causes spillover and uneven cooking, while underfilling leads to tiny, shallow donuts, so aim for consistent spoonfuls.
While the donuts cook (3-4 minutes a batch), it's time to make the glaze.: The donuts will emit a gentle, bready aroma and you may hear faint hissing as steam escapes; this is normal. Use this time to combine glaze ingredients and bring them to a simmer so they are ready to coat the hot donuts. Rushing the glaze can make it too thin or uneven, so keep a close eye on the pot.
In a small saucepan, combine all the glaze ingredients and bring to a boil, stirring constantly.: The glaze will transform from grainy to glossy as the sweetener dissolves and the mixture heats. Stirring prevents scorching and ensures a silky finish. If the glaze is not fully dissolved, it can feel slightly gritty on the finished donut, so continue until it looks uniform and shiny.
Boil for 1 minute and remove from heat.: You will see the glaze bubble gently and thicken a little, and a small whisk will leave a trail that fills in slowly when lifted. That brief boil stabilizes the glaze so it clings to the donuts, creating a thin shell as it cools. Boiling much longer risks over reducing and making the glaze too thick or grainy, so stopwatch it for accuracy.
Take your cooked donuts out of the pan and dip right into the glaze, or use a spoon—either works!: When you remove the donuts, they should be warm and spring back lightly when touched. Dipping them immediately helps the glaze adhere smoothly and creates an even coating; using a spoon allows you to control coverage if you prefer a lighter finish. A common error is letting the donuts cool too long which makes the glaze less likely to stick, so dip while warm.
DEVOUR!: The first bite should present a contrast of warm, soft interior and glossy, slightly cool glaze, with little pops from the poppy seeds . Enjoy slowly to appreciate the lemon lift and tender crumb. Be mindful that the glaze can be sticky at first, so place finished donuts on parchment as they set and avoid stacking them until the coating has slightly cooled.