Coconut mousse is a light, airy filling that softens a dense sponge and adds a clean, milky coconut flavour. The usual failures are a mousse that never sets and one that turns grainy or splits. Both come from temperature: the white chocolate is either too cool and sets in lumps, or too warm and melts the whipped cream.

Ingredients and what each one does

White chocolate is the setting agent. Its cocoa butter is solid when cold, so it turns the mousse from liquid into something sliceable without gelatin. This is a general rule for chocolate mousses.
Coconut milk carries the flavour and dissolves the chocolate. Use full-fat canned milk. A thin carton drink has too little fat and too much water, so the mousse stays soft.
Double cream is the air. Whipped cream holds thousands of tiny bubbles in a fat network, and that is what makes it a mousse rather than a pudding.
Desiccated coconut adds texture and a stronger coconut note. It also absorbs a little water, so it firms the mousse slightly.

How to make it

Heat the coconut milk until it steams, roughly 70 to 80 °C, but do not boil. The heat has to melt the cocoa butter completely, and boiling can make the milk’s fat separate. Sign: it is hot to the touch, with small bubbles at the edge. Shake or stir the milk first, because the fat rises to the top in the can.
Pour the hot milk over the chopped white chocolate and let it sit, then stir from the centre outwards. The chocolate melts into the liquid and forms an emulsion. Sign: the mixture is completely smooth and glossy, with no specks. If it looks oily, whisk it more or use a stick blender. This usually takes 1 to 3 minutes.
Cool the mixture to about 30 °C, stirring now and then. This is the most important temperature in the recipe. At that point the cocoa butter is still liquid but no longer hot enough to melt the cream, which is why the site note says 30 °C. Sign: a drop on your wrist feels neutral or only barely warm. This can take 20 to 40 minutes. A bowl of cold water speeds it up, but stir so the edge does not set.
Whip the cold double cream to soft peaks. Cold fat holds air best, and soft peaks fold in smoothly. Stiff peaks turn grainy when folded and can collapse into butter-like crumbs. Sign: the cream holds a soft peak that bends over at the tip.
Fold in the cream in two or three parts, starting with a third to loosen the base. A spatula cuts through the mixture without knocking the air out, whereas a whisk would. Sign: no white streaks remain and the mousse looks like thick, glossy pudding.
Fold in the desiccated coconut at the end. Stirring it in any earlier only adds water-soaking time.
Spread the mousse straight away in the cake, in a ring or tin, to the default height of about 2 cm, and chill until set. The cocoa butter crystallises as it cools and holds the bubbles in place. Sign: the surface no longer wobbles and a knife leaves a clean cut. Try and weigh: 4 to 6 hours is our estimate, since it depends on your fridge and the fat in your coconut milk.

What can go wrong

Lumps of white chocolate in the mousse. The base was too cool when the cream went in, so the cocoa butter set on contact. Gently rewarm the base to about 30 °C before folding.
Runny, never sets. The coconut milk was low in fat, the base was too warm and melted the cream, or the cream was under-whipped.
Grainy or split texture. The cream was whipped too stiffly or folded too hard. Whip it less next time.
Oily surface. The chocolate and milk were not fully emulsified in step 2, or the milk boiled.

Substitutions

Coconut cream instead of coconut milk gives a firmer, richer mousse, but the flavour is heavier. Whipping cream with lower fat works too, but the mousse will be softer and less stable. Add gelatin only if you plan to unmould tall slices. For a 2 cm layer inside a cake, try the chocolate alone first and weigh the result. Toasting the coconut is a matter of taste.

Storage

Covered in the fridge for up to 2 days, since the cream and coconut milk are perishable. As a cake layer, it keeps as long as the cake does. Do not freeze it unless you have tested it: coconut fat can look grainy after thawing.