With an electric mixer, combine room temperature butter and sugar for about 5-minutes at high speed until the texture is light and fluffy.
Mix in the eggs, vanilla, and mashed banana. Combine for 1-minute at a medium-low speed.
Mix in the flour, baking soda, and salt at a low speed. Combine for about 30-seconds.
Transfer batter to a greased 10 x 15 baking dish. Bake for about 25 minutes, or until the edges are lightly golden and a toothpick inserted into the center comes out clean.
Let the bars cool for about 20-minutes on the countertop.
Frosting for Banana Bread Bars
High speed, combine one cup of powdered sugar, melted butter, and vanilla.
Add in the remaining powdered sugar and three tablespoons of milk. Beat on a high speed for about 3-minutes until the frosting is light and fluffy.
While the bars are still slightly warm, evenly spread the frosting over the top. Don't worry if some of the frosting starts to melt, this will give it the cinnamon-roll style icing.
Place bars into the refrigerator for about 45-minutes!
Slice into 20 bars and enjoy!
Notes
Storing: Place them in an airtight container to keep them moist and prevent the frosting from drying out. Store them for up to 3 days at room temperature or 7 days in the refrigerator. Tip: heat banana bread bar in the microwave for 10-seconds for the best morning breakfast to go with coffee!