Preheat oven to 350ºF and line a 12-count standard cupcake tin with cupcake liners. Or if you prefer you can use a 6-count jumbo size muffin tin. If so, line it with paper liners. Set pan aside.
In a large bowl, blend the melted and cooled butter, beaten eggs, and vanilla extract together with a handheld mixer or standing mixer. Add the mashed bananas, cake flour, brown sugar, baking powder and soda, buttermilk, cinnamon and salt and blend. Use a rubber spatula or wooden spoon to fold in diced strawberries.
For the topping, in a separate bowl, add the brown sugar, butter, flour, and cinnamon and use a fork to blend everything together. It should look crumbly.
Fill the cupcake tin with the muffin batter about halfway full then sprinkle some of the crumble topping on top of each muffin. Place in the oven and bake for 20 to 25 minutes if you are using the standard size cupcake pan (makes 12). If you are using the jumbo cupcake pan (makes 6), bake for 30 to 35 minutes. Once finished baking, turn muffins onto a cooling rack and allow to cool.