Preheat oven to 180C. Lightly oil a 23cm loose bottom cake tin.
Toss sliced bananas in a little lemon juice to prevent browning.
Melt butter and sugars in a saucepan over a gentle heat, until the sugars have dissolved. This will only take a few minutes.
Add the sugar and butter (now a kind of toffee) mix to the bottom of the cake tin. Mine went slightly split at this point but it worked out fine so don't worry if yours does too.
Place the bananas on top of the toffee, creating a thin layer in the cake tin.
Lightly toast (dry fry in a pan over a low heat for a few minutes until the oils are released and the smell becomes toasty) the pecans. Then chop them roughly. Scatter these over the bananas evenly.
To make the bottom of the cake, mash the banana and mix with vanilla extract and creme fraiche. Cream the butter and sugar until pale and fluffy with an electric whisk or a wooden spoon. Add the eggs, one by one and beat some more. Now fold the flour in, and finally the banana mix.
Top the banana and nut toffee base with this cake mixture. Level off as well as possible and bake for 45 minutes.
Let cool in its tin before loosening tin and turning out. Some bananas may stick to the loose bottom - just peel them off and stick back on to the top. Dust with icing sugar if you so wish.
Serve with lightly whipped cream.