Preheat oven to 350° F. Grease an 8 ½ by 4 ½ inch loaf pan.
Whisk the flour, baking powder, baking soda, and salt together in a mixing bowl and set aside.
In another bowl, beat the Truvia and the softened butter together, then beat in the eggs and vanilla. Scrape bowl and stir in the buttermilk.
Pour the liquid mixture into the flour mixture and stir until mixed, then stir in the bananas. Do not overmix.
Pour batter into the loaf pan and bake until golden brown and a toothpick inserted into the center of the loaf comes out clean (about 55 minutes).
Let cool in pan for 10 minutes. Remove loaf from pan and transfer to a wire rack.
Notes
I measure bananas after they are mashed, since bananas come in different sizes. For this bread I used 1 ½ cups, which turned out to be 3 ⅓ large bananas.