Line a spring mould with baking parchment.
Make the caramel by putting the sugar into a pan on a low heat. Once the sugar has melted, add the butter. Whisk everything together until the butter has melted as well. Pour the mixture into your prepared baking tin.
Cut a banana into 1 cm (0.4 inches) slices and arrange them on the caramel. How many bananas you'll need exactly, will depend on the size of your tin and the size of the bananas.
Pre-heat the oven to 180°C (350°F).
In a mixer-bowl add sugar, sunflower oil and eggs. Mix until combined.
In a separate bowl, combine your flour, cornstarch and baking powder. Add this, a bit at a time, to your egg mix.
Mash the bananas with a fork before adding them to the batter.
Finally add the crème fraîche to the batter. Only mix until combined, avoid over-mixing.
Pour the batter into the baking tin and put in the oven for about an hour.
After taking your cake out of the oven, place onto a wire rack for 10 minutes. Then remove the ring from the springmould.
Put a plate upside-down on the cake and then turn the cake over. Carefully remove the baking parchment from the top.
The cake can be eaten both warm or cold.
