Notes
The cake can be baked a few days ahead and frozen. Let it thaw on the counter for hour before icing it.
If you don’t have a piping bag, you can frost the top of the cake and place the blueberries on top of the icing in the center of the cake.
If you want a less sweet icing, replace 1/4 c of the butter with 1/4 c of cream cheese.