Preheat the oven to 350°F (175°C). Grease and flour a 9-inch round cake pan.

In a mixing bowl, cream together the softened butter and sugar until light and fluffy.

Add the mashed bananas, eggs, and vanilla extract, and mix until well combined.

In another bowl, whisk together the flour, baking soda, and salt.

Gradually add the dry ingredients to the banana mixture, alternating with the buttermilk, and mix until just combined.

Pour the batter into the prepared cake pan and smooth the top.

Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted into the center comes out clean.

Remove from the oven and let cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.