Cup of Yum
1. Preheat oven to 400 degrees. Spray a 6-cavity standard donut pan with nonstick spray.
2. In a large bowl, use an electric mixer to beat together the cake mix, yogurt, water and egg until smooth. Gently fold in blueberries, if using. Evenly distribute batter into the cavities of the donut pan and smooth out the tops. I like to pipe the batter into the donut pan by transferring the batter to a large Ziploc bag and then snipping a hole in one corner.
3. Bake until a toothpick inserted into a donut comes out mostly clean, about 12 minutes.
4. Cool donuts in the pan for 10 minutes, and then transfer to a wire rack to finish cooling completely (about 15 more minutes).

PREPARE THE GLAZE:

Cup of Yum
1. Once the donuts are cool, whisk together the powdered sugar, milk, vanilla extract and salt until thick and smooth. Add a small amount of additional milk to thin, as necessary, or add a small amount of powdered sugar to make it thicker (if necessary). You want it nice and thick so that it clings to the donuts.
2. Place a wire rack over paper towels or foil (for easy cleanup). One at a time, dip the donuts in the glaze, allowing most of the excess to drip off. Place on the wire rack and allow the icing to set for about 20-30 minutes before serving.