Preheat oven to 180 degrees C/350 degrees F
Mash the bananas in a bowl and set aside.
In a large mixing bowl, add the monkfruit sweetener, vanilla and softened butter. (If butter is straight from the fridge, microwave for 15 secs to soften). Use handheld electric beaters to beat the sweetener and butter for 2 minutes
Add the eggs one at a time, using the electric beaters to mix each egg in.
In a separate bowl, add the flour, cinnamon, baking soda and bicarb soda. Mix with a spoon to combine.
To the bowl of wet ingredients, add the dry ingredients, followed by the mashed banana and sour cream.
Mix with a wooden spoon gently until the mixture is all combined and a batter is formed.
Line a loaf tin with baking/parchment paper. I used a rectangle metal loaf tin 30 cm x 15 cm (12 inch x 6 inch)
Bake in oven at 180 degrees C/350 degrees F for 50 minutes.
Remove cake from oven and test with a cake skewer. If it comes out clean, the cake is cooked
Let cake cool in the tin for 15 minutes, then take it out to further cool on a wire rack, covered with a tea towel.