Preheat an oven to 375 degrees F (190 degrees C). Grease a 9x13-inch baking dish.
Combine the milk and lemon juice in a small bowl; allow to sit at room temperature for 10 minutes. This will effectively 'sour' the milk.
Stir the 1/2 cup margarine, the white sugar, eggs, salt, and flour together in a bowl. Add the soured milk and mashed bananas and mix thoroughly; spread into the bottom of the prepared dish.
Bake in the preheated oven until the edges begin to brown very slightly, about 25 minutes. Allow to cool at least 10 minutes before frosting.
Prepare the frosting by beating together the cream cheese, confectioners' sugar, and 1/3 cup margarine in a bowl using an electric mixer. Spread evenly over the cooled bars before cutting into 20 even-sized pieces. Serve immediately or store in refrigerator.
