Preheat oven to 350F. Grease and flour three 6" cake rounds, line with parchment.
In a medium bowl, whisk cake flour, baking powder, and salt. Set aside
Beat butter until smooth. Add sugar and beat on med-high until pale and fluffy (2-3mins).
Reduce speed and add eggs one at a time, fully incorporating after each addition. Add vanilla and mashed bananas and mix until well combined.
Alternate adding flour mixture and buttermilk, beginning and ending with flour (3 additions of flour and 2 of milk). Fully incorporating after each addition. Do not overmix.
Gently fold in blueberries, being careful not to break them.
Spread batter evenly into prepared pans and bake for 30mins or until a toothpick inserted into the center comes out mostly clean.
Place cakes on wire rack to cool for 10mins then turn out onto wire rack to cool completely.