Heat oven to 160C/140C fan/gas 3. Grease a 20cm square cake tin and line the base with baking parchment. For the cake, pour in half the syrup, swirling to coat the bottom. Peel and halve the 3 ripe bananas lengthways and lay, cut-side down, in the tin.
Beat together the butter, sugar, eggs, vanilla and overripe banana with an electric whisk. Tip the flour and pecans into a food processor and pulse until finely ground together. Stir into the butter mixture with the bicarbonate of soda and cinnamon, then stir in the yogurt. Carefully spoon into the tin without dislodging the bananas. Bake the cake for 45 mins-1 hr until a skewer inserted into the centre comes out with only moist crumbs.
Meanwhile, make the sauce. Toast the pecans for 1-2 mins in a pan, add the remaining ingredients and cook until the butter has melted. Then bubble for 5 mins until it has thickened a little.
When the cake is cooked, poke it all over with the skewer – inserting it about halfway into the cake each time. Pour over the remaining maple syrup and allow it to soak in for a few mins, then turn the cake out of the tin, upside-down, onto a serving plate. Reheat the sauce, and serve with Greek yogurt or vanilla ice cream.