Berry mousse is a light, airy layer. It adds acidity and freshness between sponge and rich cream, and it must be firm enough to slice. The most common failure is a mousse that stays runny or turns dense. The real cause is almost always temperature: the purée was too warm when the cream went in, or too cold, so the gelatine had already started to set.

Ingredients and what each one does

Raspberries give flavour, colour and acidity. Their water is what the gelatine has to hold.
Gelatine turns the liquid into a gel that supports the whipped cream. It is not in the ingredient list, but without it the layer collapses when cut.
Double cream brings fat and trapped air. The air gives lightness, and the fat gives a soft, rounded mouthfeel.
Caster sugar sweetens and dissolves quickly in warm purée. Rule of thumb: too little sugar makes raspberries taste sharp and thin. How sweet is a matter of taste, so try and weigh.

How to make it

Soak the gelatine in cold water. It absorbs water and swells evenly, so it later melts without lumps. Sign: a soft, wobbly mass with no dry powder or hard centre. About 5 to 10 minutes.
Blend the raspberries with the sugar and sieve out the seeds. The sugar draws out juice and dissolves, and sieving removes grit. Sign: nothing gritty on the back of the spoon. Sieving is about texture, so if you do not mind seeds, try and weigh.
Warm a small part of the purée to about 40 to 50 °C, stir in the soaked gelatine, then mix it back into the rest. Gelatine melts at body temperature, and long boiling weakens its setting power (a general rule). Sign: no visible grains, and the purée is smooth and glossy.
Cool the purée to room temperature, about 20 to 25 °C, stirring now and then. Warm purée melts the cream’s air, and purée that is too cold sets in strings. Sign: it feels neutral on your wrist and has just begun to thicken, falling from the spoon in a slow ribbon. This usually takes 15 to 30 minutes, and an ice-water bath speeds it up.
Whip cold cream to soft peaks. Cold fat holds air, and soft peaks leave room for the extra air that folding adds. Sign: the tip of a peak bends over. Double cream goes from soft to grainy fast, so stop early.
Fold in a third of the cream to loosen the purée, then fold in the rest with a spatula, cutting down and turning over. Folding keeps the air that whisking would knock out. Sign: no white streaks, and the mousse holds a soft ribbon. About 1 to 2 minutes.
Pour at once over the sponge in the ring and level it. Chill until set. Sign: the surface does not ripple when you tilt the tin and the edge holds when cut. Usually 4 to 6 hours, or overnight.

What can go wrong

Runny, never sets. Too little gelatine for the amount of purée, gelatine boiled hard, or too much cream for the gel to hold. Our rough starting point is that gelatine should be about 1 to 1.5% of the total mousse weight, but that is our estimate, so try and weigh. Fresh pineapple, kiwi or papaya also break gelatine down with enzymes, though raspberries do not.
Rubbery strings or lumps. The purée was too cold when the cream went in, or the gelatine was not fully dissolved. Warm gently, stirring, until smooth, then cool again.
Dense, flat mousse. The purée was warm, the cream was whipped too stiff, or the mixture was over-folded. All three lose air, and the lost air cannot be restored.
Grainy, greasy texture. The double cream was over-whipped and the fat began to clump. Stop at soft peaks.

Substitutions

Frozen raspberries work. Thaw them and sieve, because they release more water, so weigh the purée instead of going by volume. Whipping cream instead of double gives a lighter, less rich mousse that is slightly less stable.

Storage

Keep it in the cake in the fridge for 2 to 3 days. It also freezes well, but the colour fades and the texture softens after thawing, so freeze only if you have to.