Preheat the oven to 350F degrees. Line mini cupcake pan with mini cupcake liners. Set aside.
In a large bowl using an electric mixer, mix bananas until mashed and creamy. Add boxed cake mix, water, oil, eggs, baking soda, and baking powder; mix together on medium speed, about 2 minutes. Mixture should be combined and relatively smooth.
Scoop batter into mini cupcake liners - no more than ⅔ full, or use a #60 disher to scoop batter. Bake for about 7-9 minutes. Check cupcakes around 8 minutes to avoid over-baking. Insert a toothpick into the center of the cupcake, if it comes out clean it is done. Remove mini cupcakes from oven and allow to cool in cupcake pan for 2 minutes; transfer to wire rack and cool completely. Cupcakes should be cooled completely before frosting.
For Strawberry Buttercream
Hull and halve strawberries. Place in a food processor and blend until strawberries are pureed; no chunks. Set aside.
In a large bowl using an electric mixer, cream butter until creamy and smooth. Add vanilla extract; mix until combined.
Turn mixer on lowest speed and slowly add 1 cup of powdered sugar; mix until creamy. Scrape down bowl as necessary. Add 1 tablespoon strawberry puree; mix until combined.
Alternate between adding 1 cup powdered sugar and 1 tablespoon strawberry puree until remaining powdered sugar and strawberry puree has been added. Blend until creamy.
Pipe or frost cooled cupcakes. Eat & Enjoy!
Notes
Store Strawberry Banana Cupcakes in an airtight container in the fridge; will keep 2-3 days.¹ For a creamy buttercream use pure cane sugar powdered sugar. If it doesn't say "pure cane sugar" it is beet sugar, which can leave buttercream gritty.² This is about 6-8 medium strawberries.Standard Cupcakes: Fill standard cupcake liners no more than ⅔ full, or use a #24 disher to scoop batter. Bake 13-15 minutes. Check cupcakes around 14 minutes to avoid over-baking. Insert a toothpick into the center of the cupcake, if it comes out clean it is done. Yields about 24 standard cupcakes.