Preheat your oven to 350°F. Grease and flour three 6-inch cake rounds, then line the bottoms with parchment paper. In a medium bowl, whisk together the sifted cake flour, baking powder, and salt; set aside. In a large bowl, beat the butter until smooth. Gradually add the sugar and beat on medium-high speed for 2-3 minutes until the mixture is light and fluffy. Reduce the mixer speed and add the eggs one at a time, making sure each egg is fully incorporated before adding the next. Stir in the vanilla extract and mashed bananas until well combined. Alternate adding the dry flour mixture and buttermilk to the wet ingredients, starting and ending with the flour mixture (three additions of flour and two of buttermilk), mixing fully after each addition but being careful not to overmix. Gently fold in the floured blueberries to avoid breaking them. Divide the batter evenly among the prepared pans and bake for about 30 minutes or until a toothpick inserted into the center comes out mostly clean. Let the cakes cool in their pans on a wire rack for 10 minutes before turning them out onto the rack to cool completely.

Using a stand mixer or hand mixer, beat the butter until pale and creamy. Lower the speed and add the powdered sugar one cup at a time, mixing on low until well combined after each addition. Increase the speed to medium and beat for 3 minutes. Add the vanilla extract and continue to beat for another minute. Slowly add the chilled, cubed cream cheese one tablespoon at a time, beating well after each addition until the frosting is smooth and free of lumps. If the frosting is too soft, optionally add cornstarch or meringue powder one tablespoon at a time to stiffen it. Finish by mixing on low speed for a couple of minutes to remove any air bubbles.

Place one cake layer on a serving plate or cake stand and spread about ⅔ cup of frosting evenly over the top. Repeat with the remaining layers, then apply a thin crumb coat over the entire cake. Chill the cake for 20 minutes to set the crumb coat. Use the remaining frosting to cover the cake completely, creating rustic swirls on the sides and top with an offset spatula if you like. For a finishing touch, decorate with fresh blueberries and sprigs of thyme, if desired.