• Preheat oven to 350 degrees (F).
• Lightly grease a doughnut pan; set aside.
• In a large bowl whisk together the dry ingredients. In a separate bowl whisk together the sour cream and egg, gently fold them into the dry mixture. Pour brown butter into the mixture and fold together until just combined.
• Spoon mixture into prepared doughnut pan, and bake for 10-12 minutes, or until the doughnuts spring back when lightly pressed. Allow doughnuts to cool a few minutes in the pan before transferring to a wire wrack to cool for a few more minutes. Once cool to the touch, dip each donut in the melted butter, then roll them in the cinnamon/sugar mixture. Return them to the wire until needed.

For the caramel:

• Combine the butter, heavy cream, sugar, and salt in a heavy bottomed saucepan. Place pan over medium-high high, and, stirring constantly, bring to a boil. Be sure to break up any chunks of sugar with the whisk! Once the mixture has reached a boil, stir well once more, reduce the heat to a simmer, and cook for 8 minutes, stirring occasionally. Add bourbon, if using, and vanilla - carefully, as the mixture will bubble up as you do this - whisk together, and boil for another 2 minutes. Remove from heat and pour mixture into a heatproof glass container.

For Assembly:

• Place a donut on a small serving plate or bowl. Top with a few scoops of ice cream, some banana slices, walnuts, and caramel. Serve at once!