Preheat the oven to 275 degrees F (135 degrees C). Grease and flour a 9x13-inch pan.

Make cake: Mix mashed bananas with lemon juice in a small bowl. Mix flour, baking soda, and salt in a medium bowl.

Cream sugar and butter in a large bowl until light and fluffy, 3 to 4 minutes. Beat in eggs one at a time, then stir in vanilla. Beat in flour mixture alternately with buttermilk. Stir in banana mixture. Pour batter into the prepared pan.

Bake in the preheated oven until a toothpick inserted into the center of the cake comes out clean, about 1 hour. Remove cake from the oven and place it directly into the freezer for 45 minutes. This will make the cake very moist.

Meanwhile, make frosting: Cream butter and cream cheese until smooth. Beat in vanilla. Add confectioners sugar and beat on low speed until combined, then on high until frosting is smooth.

Spread frosting on cooled cake.