In a small bowl, whisk together the flour, sugar, baking powder, baking soda and salt. In another small bowl, mash the banana into the buttermilk with a fork, then stir in the caramel and vanilla until blended. Mix the mashed bananas into the flour mixture, and add the melted butter. Stir until just combined, then divide into two 7.5 oz mugs. If you use a different sized mug, make sure to leave room for the cakes to rise.
Place the mugs on a paper towel inside the microwave, and cook for about 120 seconds on high, then continue cooking in 20 second increments until the cake has risen, is fluffy and firm, and fully baked. The cook time will vary depending on the wattage of your microwave. For my 1200W microwave, two mug cakes cooked at the same time took about 200 seconds total (working in shorter increments towards the end really helps you fine tune the bake). Please keep in mind that your microwave may take less or more time.
Let the cakes cool for several minutes, then serve with whipped cream and additional caramel sauce.