Preheat oven to 350 degrees F. Grease a 9×13-inch metal baking pan or line with parchment paper. Set aside.
In a large bowl, whisk together the flour, baking soda, baking powder, cinnamon, and salt. Set aside.
In the bowl of a stand mixer (or with a hand mixer), beat the butter and sugar together until creamy and smooth, about 3 minutes. Stopping to scrape down the bowl with a spatula, as necessary.
Beat in the eggs, one at a time, and add the vanilla extract. Add the mashed bananas and mix until combined. Scrape down the sides of the bowl with a spatula, if necessary.
With the mixer on low, slowly add the dry ingredients in three rounds, alternating with the buttermilk, until completely combined. You want to end with the dry ingredients. Don’t over mix.
Spread batter into the prepared pan and set aside while you make the cinnamon filling.
In a medium bowl, stir together the softened butter, brown sugar, flour and cinnamon until creamy and smooth.
Drop small dollops all over the top of the batter. Using the tip of a butter knife swirl the filling into the top of batter.
Bake the cake for 30 to 35 minutes or until a toothpick inserted into the center of the cake comes out clean. Every oven is different so check the cake early to be safe.
Remove the cake from the oven and set the pan on a wire rack. Let cool for 10 to 15 minutes. Don’t let it cool completely, you want the cake to be warm when you add the glaze.
Make the glaze. Using a stand mixer or hand mixer, beat the butter and cream cheese until smooth. Add the confectioner’s sugar, vanilla, and 2 tablespoons of milk; beat for 2 minutes or until smooth. If the glaze is too thick, gradually add more milk, 1 teaspoon at a time, until desired consistency is reached.
Spread the glaze over the warm cake. Cut into squares and serve.