Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking dish.
Mash the bananas in a small bowl with the lemon juice. Set that aside.
In another bowl, whisk together the flour, baking soda, and salt.
In a large bowl, beat the butter and sugar until light and fluffy. Add eggs one at a time, mixing well after each.
Stir in the vanilla extract.
Add the flour mixture to the butter mixture alternately with the buttermilk. Mix just until combined.
Fold in the mashed banana mixture.
Pour the batter into the greased pan and smooth the top.
Bake for 60–70 minutes, or until a toothpick comes out clean. Let it cool completely before slicing.