Heat oven to 350 F.

In large bowl, whisk cake flour, all-purpose flour, baking soda and baking powder.

In large bowl or stand mixer bowl, beat butter, sugar, 1/8 teaspoon salt and 1 teaspoon vanilla until fluffy. Add eggs and yolk; mix until creamy. Alternate adding buttermilk and flour mixture until blended.

Remove bowl from stand mixer, add blueberries and stir until combined.

Fill piping bag with batter. Fill mini donut pan cavities about halfway.

Bake 8-10 minutes. Cool completely.

In medium bowl, whisk 1 cup powdered sugar, milk, remaining salt and remaining vanilla extract. Add additional 1-3 cups powdered sugar until mixture reaches desired thickness.

Dip cooled donuts in glaze and place on wired rack to drip off excess.