Grease and line a 20cm square tin with baking parchment and preheat oven to 180°C.
Melt butter and sugar in a small pan over medium heat until slightly thickened.
Pour the mixture into the prepared tin and spread evenly before it solidifies.
Toast the pecans in the oven for five minutes and allow to cool.
Cream butter and sugar until fluffy, then beat in eggs and vanilla.
Fold in both flours, baking powder, spices, treacle, and salt.
Arrange 5mm banana slices in a tight layer over the caramel in the tin.
Drizzle calvados over bananas and stir chopped pecans into the cake batter.
Spread cake batter over the bananas and bake for approximately 45 minutes at 180°C.
Check with a skewer after 35 minutes to ensure it comes out clean.
Cool in the tin for 10 minutes then flip onto a plate and remove parchment.
Cut into squares and serve with cream or ice cream.