Preheat the oven to 325 degrees F.
Grease and flour a large bundt pan and set aside.
In a medium size mixing bowl, add the flour, baking powder, baking soda, salt and cinnamon. Whisk well and set aside.
In a large mixing bowl, add the room temperature butter, brown sugar and granulated sugar and mix with an electric mixer on medium-high to high for approximately 2 minutes.
Add the eggs one at a time, mixing on medium for about 15-20 seconds after each addition.
Scrape down the bowl and add the mashed bananas and vanilla, then mix on medium speed until well combined.
Alternately add the flour mixture and sour cream to the butter/sugar mixture as follows: Start with adding a third of the flour mixture to the butter/sugar mixture and mix on medium speed only until just combined (about 10-15 seconds only).Add half of the sour cream and mix on medium speed only until just combined (about 10-15 seconds only).Add another third of the flour mixture and mix on medium only until just comined (about 10-15 seconds only).Add the last half of the sour cream and mix on medium only until well combined (about 20 seconds only).IMPORTANT NOTE ABOUT SCRATCH CAKES: Be very careful only to mix until well combined. Once the batter is well combined, stop mixing. The batter does not have to be silky smooth. Mixing scratch cake batter too long and/or at too high of a speed can cause the gluten in the flour to over-develop and that will cause the cake to come out dense, rubbery or it may fall in the middle. Scrape down the sides of the bowl and mix again, but only by hand with a large spoon or a silicone spatula.
Add chopped nuts if desired and mix those in by hand.
Pour the batter into the prepared pan and bake at 325 degrees F for approximately 55 - 65 minutes. Ovens vary. The cake is fully baked when a toothpick inserted into the cake comes out with a few moist crumbs on it or clean, but no raw batter should appear.
Set the baked cake on a wire rack, still inside of the pan, to cool for 15 minutes, then turn the cake out of the pan and onto the rack and let cool completely.
While the cake is cooling, prepare the glaze.