Cream the butter and sugar together in a large bowl until light and fluffy. A small electric hand mixer does this just fine. Add the 2 eggs and mix well until you have a smooth light airy mixture.
Mash the bananas one at a time and add them. Mix well between each banana until they disappear into the mixture. These hold the muffins together really well and allow this recipe to be made without xanthan gum or another binding ingredient.
Mix the rice flour, cornflour, baking powder and salt together in a separate bowl. Sift this mix into the butter and egg mixture bit by bit mixing in between until you have a thick but even consistency and all the flour mix has disappeared into the wet mix. Fold in the sultanas gently.
Spoon into muffin cases until even. The baking powder will make them rise so don't make them overflowing.
Bake for 20-25 minutes until golden brown. Allow to cool for 5 minutes.