Preheat oven to 350 degrees. Grease (I use cooking spray) and flour one bundt pan.
In a large bowl, cream the butter with sugar until light and fluffy. Add the eggs and mix well.
Whisk together the flour, baking soda, salt, cinnamon and cardamom; combine with the butter mixture.
Add the sour cream, bananas, and vanilla; stir well.
Pour into prepared bundt pan and bake for 35-40 minutes. Use a toothpick to see if the cake is done. If it comes out clean, it’s done.
Cool for 10 minutes and then turn the cake out onto a rack.
Frosting:
Beat with mixer for about 1 minute the butter, mashed bananas, lemon juice, and vanilla. Add in the powdered sugar.
Immediately spread onto the banana cake. If you want it thinner, add a few drops of milk. If you want it thicker, add a bit more powdered sugar.
Toast the walnuts; chop in big chunks and sprinkle on the cake. [I toast on a papertowel in the microwave for 1 minute; toss around, and then another 30 seconds.]