Preheat oven to 350 degrees F. Line a cupcake pan with paper liners. In the bowl of an electric mixer fitted with the paddle attachment, beat the butter and sugar until light and fluffy. Scrape down the sides of the bowl with a rubber spatula and continue to beat, adding one egg at a time until thoroughly combined. Add the sour cream and bananas and mix until combined.
In a medium bowl, sift together the flour, baking soda, cinnamon, and salt. Add flour mixture to the mixer and beat until just smooth, 1-2 minutes. Add the vanilla and beat until just combined.
Fill each cup about 2/3 of the way full with the batter and bake, rotating halfway through, until the cupcakes are golden and a cake tester inserted int he middle comes out clean, 18-20 minutes. Let cupcakes cool in the pan for 5 minutes, then transfer to wire rack to cool completely before frosting.
Spoon about 2 tablespoons of the frosting onto each cupcake. Using an offset spatula and holding the cupcake in your hand, spread the frosting on in swirls by turning the cupcake in your hand. Ring each cupcake with pastel candies or sprinkles, if using. Serve immediately or store in an air tight container and refrigerate up to 3 days.