Preheat the oven to 350°F (175°C). Grease a 8-inch heart-shaped cake pan and lightly flour it, tapping out excess flour.
In a medium bowl, whisk together the flour, baking soda, and salt. Set aside.
In a large bowl, beat the softened butter and granulated sugar together with an electric mixer on medium speed until light and fluffy, about 3 minutes.
Add the eggs one at a time, beating well after each addition. Mix in the vanilla extract.
Add the mashed bananas to the butter mixture and stir until combined.
Alternately add the flour mixture and buttermilk to the banana mixture, beginning and ending with the flour mixture. Mix until just combined; do not overmix.
Pour the batter into the prepared heart-shaped cake pan and smooth the top with a spatula.
Bake for 35 to 40 minutes, or until a toothpick inserted into the center comes out clean.
Remove the cake from the oven and allow it to cool in the pan for 10 minutes.
Run a knife around the edges of the pan to loosen the cake, then invert it onto a wire rack to cool completely before serving.