Caramelised banana is a soft, fruity filling that adds sweetness and a toasty edge to a plain sponge. The most common failure is a grey, watery mush. The cause is rarely the recipe. Usually the banana stayed on the heat too long, or the filling went into the cake while still warm.

Ingredients and what each one does

Bananas give flavour and body. They are roughly three-quarters water, and that juice loosens the caramel. Ripe ones, yellow with brown freckles, are best. As bananas ripen, starch turns into sugar, so they taste stronger and soften evenly.
Caster sugar becomes the caramel. It melts evenly and quickly without much stirring, because the crystals are fine.
Butter smooths the caramel and gives gloss. It also cools the caramel a little before the fruit goes in.

How to make it

Slice the bananas to about the thickness of the finished layer. Mechanism: slices of even thickness soften at the same rate. Sign: they feel firm, not slimy. Cutting thicker or thinner is a matter of taste, so try and weigh.
Put the sugar in a dry, wide pan over medium heat. Do not stir until the edges melt, then swirl or stir gently. Mechanism: stray crystals on the pan wall can seed crystallisation across the whole batch. Sign: deep amber colour and a smell that turns from sweet to toasty. This usually takes 5–10 minutes. Darker means bitter.
Take the pan off the heat and add the butter in pieces. Mechanism: butter melts into the caramel as a fat emulsion and lowers its temperature. It contains water, so it spits, and you should stand back. Sign: smooth, glossy, with no oily streaks.
Add the banana, return to low heat and turn gently with a spatula. Mechanism: cold fruit makes the caramel seize into lumps, and the banana’s own juice dissolves them again. Sign: the caramel is smooth and coats the slices, and the slices still keep their edges. This takes 1–2 minutes. Longer heat breaks down the cell walls of the fruit, which is a general rule for any fruit, and the slices collapse into purée.
Spread the filling on a tray and cool it completely. Mechanism: caramel thickens as it cools, and a warm filling would melt the cream and sweat condensation into the sponge. Sign: no warmth when you touch the underside of the tray. Allow 30–60 minutes at room temperature, then chill if you are not using it straight away.

What can go wrong

Mush. Overripe bananas, heating beyond 2 minutes, or hard stirring. Next time, stop earlier and fold rather than stir.
Watery filling that soaks the sponge. The juice was not absorbed by the caramel. Make sure the caramel was cooked to deep amber, which leaves less water in it. Also pipe a cream border around the layer so it acts as a dam.
Hard lumps that stay. The caramel seized and was not given time to dissolve. Keep it on low heat a little longer, but only until the lumps soften.
Bitter taste. The sugar went past amber. Colour comes before time, so watch the pan.
Slices turn brown. Bananas darken when exposed to air. Lemon juice is often added to stop it. In a caramel the colour is hidden anyway, so it hardly matters. Add a little for taste if you like, but try and weigh.

Substitutions

Brown sugar instead of caster. It gives a softer, more molasses-like caramel, but the amber colour sign is harder to read.
Firmer, less ripe bananas. They hold their shape better but taste flatter, because the starch has not yet turned to sugar.

Storage

Covered in the fridge, it is best within 2–3 days. This is our estimate, so try and weigh. The slices darken and soften with time, and the filling does not freeze well. Bring it to a cool room temperature before using it so it spreads easily.