In small bowl, using your fingers rub the lemon zest into the sugar until completely combined. The sugar will feel slightly moist as the zest releases its oil. Stir the lemon-sugar into the flour mixture.
113 grams granulated sugar, 1 teaspoon lemon zest
In a small bowl (you can use the bowl from the lemon-sugar!), whisk together buttermilk, melted butter, egg, and vanilla. Scrape into the flour mixture and use a rubber spatula to gently combine all ingredients.
In a small bowl, toss the blueberries with 1 tablespoon of flour. Gently fold the floured blueberries into the batter.
141 grams blueberries, 1 Tablespoon cake flour
Scrape batter into a piping bag or large plastic bag. Cut a hole large enough for the blueberries to pass through. Place donut pan onto the baking sheet. Pipe batter into the donut pan.
Place baking sheet with donut pan into the oven and bake for 14-18 minutes.
Remove from oven to a wire rack and allow to cool completely.
Lemon Glaze
For lemon glaze, whisk together powdered sugar and lemon juice until completely combined. Drizzle in milk only as needed to get to a thick but flowing consistency.
For blueberry glaze, whisk together powdered sugar and blueberry jam until combined. Drizzle in milk only as needed to get to a thick but flowing consistency.
The instructions state to grease your donut pan before preparing the batter. If you are going to use cooking spray, I suggest waiting to spray the pan until right before you pipe the batter in. Cooking spray tends to drip down and pool in the wells of the pan, whereas greasing with butter will not.
I like to buy 2 pints of blueberries so I can pick through and find the small and medium berries. Some are very large and will overwhelm the donut.
If you don’t have blueberry jam, you can make a small amount of your own in the microwave. In a microwave safe bowl, place 2 tablespoons of blueberries, 2 teaspoons of sugar, and 1 tablespoon of water. Microwave on high for 20 seconds. Smash the blueberries with the back of a spoon. Push everything through a sieve. Discard solids and allow jam mixture to cool.