Preheat oven to 180°C. Grease and line a 28 x 12cm loaf pan with baking paper.
Combine sour cream, thickened cream and bicarb in a large bowl. Set aside for 10 minutes or until bubbles appear on the surface. Add butter, sugar and eggs, and stir to combine.
Add cinnamon, flour and baking powder, then add choc chips, thawed bananas and half the walnuts. Swirl through Top’n’Fill caramel. Pour the mixture into pan and bake for 1 hour or until a skewer inserted into the centre comes out clean (cover loosely with foil if browning too quickly).
Cool for 10 minutes, then turn out onto a wire rack to cool completely.
Combine icing sugar and espresso until soft icing forms. Drizzle over banana bread and top with remaining walnuts.