Preheat the oven to 425 degrees
Prepare your donut pan by greasing the trays lightly with butter. Set aside.
Add butter, vegetable oil, and sugars to a large bowl and beat with a hand mixer to combine (about 3-5 mintues).
Continue to mix while adding egg, baking powder, baking soda, nutmeg, cinnamon, salt, vanilla extract, and lemon.
Slowly stir in the cake flour and buttermilk. Your mixture will be fairly thick.
Add the mixture to a pastry bag or a large ziplock bag, and snip off one corner of the bag.
Pipe the mixture into the pans, filling only about ⅔ of the way to the top of each opening.
Add blueberries to each donut by placing them in the pan and pushing them into the batter with a toothpick or spoon.
Bake donuts in pan for 8-10 minutes (you’ll know they’re done when they spring back after poking them with your finger).
Let donuts cool in pan for 8 minutes.
Meanwhile, make the lemon glaze. Add all ingredients to a bowl and stir until well combined. You can make your glaze thicker or thinner by adjusting the ratio of the amount of milk to sugar. More milk and less sugar will make your glaze a bit runnier and less milk, more sugar will make it thicker. (Note: This is glaze is also great to use as a frosting for sugar cookies! I usually omit the lemon extract and just use a whole teaspoon of vanilla when using it for sugar cookies.)
Remove the donuts from the pan, and allow to cool completely.
Dip donuts into the glaze and allow to dry. The glaze will harden to a beautiful, smooth, frosting.