Preheat the oven to 350°. Prepare a 13x9 baking dish with butter and a light dusting of flour.
In the bowl of a stand mixer or using a hand beater, cream the butter until soft and light. Gradually add the maple syrup and beat on medium speed for 5 minutes more, or until very light. The butter and maple syrup may separate slightly. Beat in the eggs, one at a time, beating well after each addition. Beat in the vanilla.
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Add to the butter mixture along with the milk and bananas. Beat on low speed just until combined.
Bake for approximately 30-35 minutes, or until a toothpick inserted in the center comes out clean. Cool in the pan for 10 minutes, then transfer to a cooling rack to cool completely.
After cake is completely cooled in a medium bowl, beat the cream cheese until smooth. Add the butter, maple syrup, and vanilla and beat until smooth. Frost cake and garnish with walnuts.