Lemon mousse is a light, tart layer that cuts through sweet sponge and rich buttercream. The common failures are a grainy base and a runny mousse. The first comes from overheated eggs. The second comes from folding cream into a base that is still warm.

Ingredients and what each one does

Lemons supply juice (acid and flavour) and zest (aroma). The aroma sits in the oils of the zest, not in the juice.
Eggs thicken the base. Their proteins unfold when heated and link up, which turns the liquid into a cream.
Caster sugar sweetens, softens the acid, and dissolves quickly. It also slightly raises the temperature at which the eggs set, so the base is a little more forgiving.
Double cream is the airiness. Whipped, its fat globules hold air bubbles, and that is what makes it a mousse rather than a curd.

How to make it

Rub the zest into the sugar with your fingertips. This breaks the oil glands in the zest, and the sugar crystals soak up the oil. Sign: the sugar looks damp, yellowish and strongly scented. About 1 minute.
Whisk the eggs, the zest sugar and the juice in a heatproof bowl set over barely simmering water, or in a pan over very low heat. Stir constantly. Gentle heat lets the egg proteins link up gradually instead of clumping. Sign: the base coats the back of a spoon and a finger drawn through it leaves a clean line. This is roughly 78 to 82 °C and usually takes 6 to 12 minutes. Do not let it boil, because above about 85 °C the proteins tighten into scrambled-egg bits.
Strain the base through a sieve. This catches any bits of cooked egg and pieces of zest. It is not strictly required, but it is the difference between smooth and almost smooth.
Cool the base with cling film pressed onto the surface, so no skin forms. A warm base melts the fat in the cream and the mousse collapses. Sign: the base feels cool, not warm, on the inside of your wrist, and is still loose enough to stir. This is about room temperature, 20 to 22 °C, and takes 30 to 60 minutes in the fridge. If it chills until stiff it forms lumps when mixed.
Whip the cold double cream to soft peaks. Whipping packs the fat globules around the air bubbles, and folding adds a little more whipping, so stop early. Sign: when you lift the whisk the peak bends over at the tip. This takes 2 to 4 minutes with a hand mixer. Double cream is high in fat and goes from soft peaks to grainy very fast.
Stir a third of the cream into the base to loosen it, then fold in the rest with a spatula, cutting down through the middle and turning the bowl. Folding keeps most of the air. Sign: no streaks, and the mousse falls from the spatula in a soft ribbon.
Spread it at about 2 cm and chill. The fat in the cream firms up and the mousse holds its shape. Try and weigh: at least 4 hours is our estimate, and an overnight chill is safer for neat slices. A ring of firmer cream around the edge keeps it from squeezing out when the next sponge goes on.

What can go wrong

Grainy, with small lumps. The eggs got too hot, or the base was chilled too hard before folding. Strain next time, and stop cooking at the coating-spoon sign.
Runny, weeping mousse. The base was still warm, the cream was under-whipped, or the mix was folded too long and lost its air.
Buttery flecks. The cream was over-whipped. Fold it in at soft peaks instead.
Bitter or metallic. Pith got into the zest, or the base was cooked in an aluminium pan, which reacts with acid. Use stainless steel or a glass bowl.

Substitutions

Lime instead of lemon works the same way. The flavour is sharper and more bitter.
Single or whipping cream instead of double gives a lighter but less stable mousse, because there is less fat to hold the air. Whip it more carefully and chill it longer.
Gelatine is not in this version. If you want sliceable, firm layers, a small amount dissolved into the warm base is a common fix. Treat the quantity as try and weigh, because too much turns the mousse rubbery.

Storage

Covered in the fridge for 2 to 3 days, since it contains cooked eggs and cream. Do not leave it at room temperature for long. Freezing can make it weep when it thaws, so try and weigh before relying on it.