Make the crumb topping first, as it will need to rest in the refrigerator for a while.
1. Mix the flour, baking powder, and cinnamon in a medium bowl.
2. Melt the butter in a pan on the stove. Remove from heat and mix in brown sugar until smooth.
3. Pour the butter-sugar mix into the bowl of dry ingredients and mix.
4. Place the crumb topping in the refrigerator for 30 minutes or more.

Muffins Mixture

1. Preheat the oven to 350 degrees F/180 degrees C and line a muffin/cupcake tin with 12 paper liners.
2. Sift the flour into a large bowl. Add all remaining dry ingredients, including the diced cashews, and mix well.
3. Combine all wet ingredients, including the mashed banana, in a separate bowl.
4. Melt the butter and add it to the wet ingredients. Blend with hand or stand mixer until thoroughly combined.
5. Add wet ingredients to the bowl of dry ingredients. Blend with hand or stand mixer until thoroughly combined.
Bake
1. Spoon muffin mixture into the paper liners. Fill each one roughly ¾ of the way.
2. Add 2 tbsp of crumb topping to each filled liner.
3. Bake for 25-30 minutes. The tops will be brown and a toothpick will come out clean when inserted in the middle of the muffin.
4. Cool for 10 minutes before handling. Remove from the pan and enjoy.