Blueberry Cake Donut Bars

For the donut bars:

• 1/2 cup butter, softened
  
• 1 cup sugar
  
• 2 1/4 cups all-purpose flour
  
• 2 teaspoons baking powder
  
• 3/4 teaspoon baking soda
  
• 1 pinch salt
  
• 2 eggs
  
• 1 egg yolk
  
• 1 cup buttermilk
  
• 1 teaspoon vanilla
  
• 2 1/2 cups blueberries
  

For the buttermilk glaze:

• 2 cups powdered sugar
  
• 3 tablespoons buttermilk
  
• 1/3 cup whole milk
  

1. With a wooden spoon, beat the butter and sugar together in a large bowl until well-combined.
2. In a large bowl, whisk together the flour, baking powder, baking soda, and salt.
3. In a medium bowl, whisk together the eggs, egg yolk, buttermilk, and vanilla.
4. Alternate adding the dry and wet ingredients to the butter mixture alternately, in 2 or 3 additions each. Mix until just combined.
5. Fold in the blueberries. Pour the mixture into a lightly greased 9- by 9-inch cake pan, then bake in a 350° F oven until a toothpick inserted into the center comes out clean, 20 to 30 minutes. Cool completely, then cut into 8 bars.
6. To make the glaze, whisk the powdered sugar, buttermilk, and milk together to combine. Make sure the glaze is runny. Add additional milk, 1 tablespoon at a time, if needed.
7. Place the bars on a wire rack and pour the glaze over top. Letit set completely before serving.