Preheat oven to 350F. Lightly grease the sides of two 6-cup donut pans. Set aside.
In a large bowl, whisk mashed banana, sugar, butter, milk, egg, and vanilla until smooth. In a separate bowl, combine flour, baking soda, baking powder, salt, and cinnamon. Fold flour mixture into liquid until batter is just combined. Set aside.
Caramel
In a medium saucepan over medium-high heat, melt butter then whisk in sugar, and agave. Bring sauce to a simmer and stir occasionally until thickened and golden. Reduce heat to medium and add cream and salt and cook for 1 more minute.
Remove caramel from heat and immediately spoon about a tablespoon into the bottom of prepared donut pan cavities. Spoon batter over caramel filling them each 3/4 to the top. Smooth tops.
Bake donuts at 350F 15-20 minutes or until caramel is bubbly and donut springs back when lightly touched. Run a knife around sides of donuts and immediately invert donuts onto a cooling rack. Serve warm and enjoy!