Preheat your oven to 160C Fan and prepare your tin. My tin is about 8 inches x 12 inches, so the popular 9x13 tin would be fine I'm sure.
If you are making your own buttermilk - put a tbsp of lemon juice in a jug and then fill it up to 270ml with milk (any milk will work - if you can have dairy, whole milk works best but I've made it with almond milk and its perfect too). Put your buttermilk to one side until you need it.
In a large mixing bowl place your butter/hard margarine and cream it / beat it using a electric hand mixer. Do this for about 30 seconds until it's a little lighter in colour.
Add both your caster and light brown sugar and mix again until completely creamed and combined.
Add your eggs and vanilla extract and mix again. (I add my eggs one at a time and briefly mix between each.)
If you haven't already, mash your bananas. Your bananas should be ripe enough that you can easily mash them with a fork. Add them to your mixture and mix briefly until combined - don't worry about lumps!
Place all your dry ingredients (gluten free flour, xanthan gum, baking powder, bicarbonate of soda and cinnamon) in a bowl and mix together with a spoon.
Add about a quarter of your dry ingredients to your cake mix and mix in. Then follow that with a quarter of the buttermilk and mix it in. Continue to do this until all the dry ingredients and buttermilk are mixed in. The mixture should be relatively thin and there's quite a lot of it!
Spoon / pour your mixture into the prepared baking tin.
(Optional) If you want to place bananas on top of the cake, cut some bananas in half, sprinkle the tops of them with light brown sugar (helps with caramelisation) and place onto the cake - don't push them down.
Place in the oven for 50-55 minutes until golden and cooked through (check with a skewer).
Allow to cool in the tin and then carefully lift it out (I usually ask someone to help as I'm useless!!). Slice into squares and enjoy!