Preheat the oven to 350 degrees and prep an 8x8 baking pan with pan spray and parchment paper.
Ina small saucepan, brown the butter. Remove the saucepan from heat and pour browned butter into a large heat safe bowl.
Add the brown sugar to the bowl and stir well. Add egg and stir until smooth then add your sour cream and mix until everything is incorporated.
In a small bowl, mash your bananas and add them to brown sugar mixture. Add your vanilla and stir well.
Ina medium bowl, add the flour and salt and whisk together. Add the flour mixture to the wet mixture and stir together until the flour is just incorporated.
Pour your batter into the prepared baking pan and bake in the oven for 20-24 minutes. Test the middle with a toothpick for doneness, but leave the middle a little under done.
Remove from the oven and let cool. While waiting for the blondies to cool, make the frosting.
In a medium saucepan, melt butter and then add brown sugar and heavy cream. Cook until the sugar is incorporated and the mixture is just about to boil. Remove from heat and let sauce cool for about 5 minutes. Add powdered sugar and mix until smooth.
When the blondies are cooled, you can frost them. Let the frosting set, cut and enjoy!