For the cupcakes: Preheat the oven to 325 degrees F. Line 2 cupcake baking pans with paper liners.
Sift together the flour, baking powder, baking soda and salt in a bowl; set aside. Beat the sugar, butter and pureed avocado until completely combined in the bowl of a stand mixer. Beat in the vanilla, then beat in the eggs one at a time, mixing well after each addition.
Beat in the buttermilk with the mixer on low speed until just combined. Add the flour mixture. Once all the flour is incorporated, increase the mixing speed to medium and beat for 20 seconds longer. Be careful not to over-mix.
Divide the batter among the pans and bake until golden on top and a wooden toothpick inserted in the center comes out clean, about 25 minutes. Cool in the pans on wire racks for 10 minutes before inverting, removing papers and cooling completely.
For the buttercream frosting: Beat the powdered sugar and egg whites for about 5 minutes on medium-high speed using an electric mixer. Puree the avocado and lime juice in a food processor. Add the butter to the whites mixture and beat until very creamy, about 10 minutes, then add the avocado and lime juice and mix well.
For the streusel: Cream together the butter, sugar and almond paste using an electric mixer. Add the vanilla and cinnamon and mix well. Add the flour and toasted oats in batches until large crumbs start forming.
For the caramelized bananas: Make a dry caramel by pouring the sugar in an even layer into a heavy-bottomed saucepan. Heat over medium heat until the caramel starts to melt, gently pushing the sugar towards the center. Continue cooking, stirring only very occasionally, until the caramel turns a dark copper, taking care that the caramel doesn't burn. Transfer to a bowl, stir in the bananas until coated and caramelized, and then stir in the butter, cream and cinnamon.
For assembly: Remove the centers of the cupcakes using a melon baller and fill with the caramelized bananas. Pipe the avocado-lime frosting on top, sprinkle with the oats streusel and garnish with lime zest on top.
This recipe was created by a contestant during a cooking competition. It has not been tested for home use.