Pistachio cream

Pistachio cream is a soft, rich layer that carries the nut flavour and keeps the sponge moist. The most common failure is lumps of paste or a grainy texture. The first comes from adding thick paste to cold, soft ingredients all at once. The second comes from overworking the mascarpone.

Ingredients and what each one does

• Pistachio paste gives the flavour and colour. It is mostly nut oil and ground nut, with very little water, so it is dense and sticky. The colour of the finished cream depends on the paste. Pure pistachio is a muted olive-green, and a bright green usually means colouring. Which you prefer is a matter of taste.
• Mascarpone is the body of the cream. It is soft and high in fat, so it blends with the paste without making it watery.
• Double cream is whipped and folded in at the end. Its fat traps air and makes the cream lighter. Without it the cream would be heavy and dense.

How to make it

1. Stir the jar of paste first. The oil rises to the top while it sits, and a cream made from the oily top and the dry bottom will not be even. Sign: the paste is uniform and glossy, with no oil layer. Use it at room temperature, because cold paste is stiffer and lumps more.
2. Loosen the paste with part of the mascarpone, roughly a third. Mascarpone is much softer than the paste, so a small portion thins it gradually instead of shocking it. Mix with a spatula. Sign: a smooth, streak-free mass that falls off the spatula.
3. Add the rest of the mascarpone in two portions and stir just until combined. Mascarpone is easy to overwork: past a point its structure breaks and it turns grainy (a general rule). Sign: a smooth, thick, glossy cream with no paste streaks. Keep the mixer on low, or use a spatula.
4. Whip the cold double cream to soft peaks. Cold fat holds air better, and soft peaks fold in without becoming grainy. Sign: the tip of the peak bends over when you lift the whisk. This usually takes 2–4 minutes with a hand mixer.
5. Fold the cream in two parts. Add a third to lighten the base, then fold in the rest gently so the air stays in. Sign: no white streaks, and the cream holds its shape on the spatula.
6. Chill it before using. The fat firms up as it cools and the cream becomes easier to spread. Sign: it keeps a soft peak and does not slump from the spatula. Chilling usually takes 30–60 minutes (try and weigh, it depends on your kitchen).

At a layer height of 1 cm the cream is thick enough to taste distinctly, but it is still soft. If you pair it with a liquid filling, pipe a rim of the cream around the edge to hold it in.

What can go wrong

• Lumps of paste. The paste went into cold mascarpone all at once. Next time loosen it with a small portion first. For this batch, push it through a sieve or warm the bowl slightly while stirring.
• Grainy, curdled texture. The mascarpone was overmixed, or whipped on high speed. It cannot be fully fixed, so use less mixing next time. Folding in a little extra softly whipped cream can mask mild graininess.
• Runny cream. The cream was under-whipped, or the paste was warm. Chill the cream for a while and see if it firms up. If it doesn’t, whip a little extra cream to soft peaks and fold it in.
• Heavy, stiff cream. Too much paste, or cream whipped too far before folding. Taste as you go and weigh how strong you want the flavour.

Substitutions

• Cream cheese instead of mascarpone. The cream will be tangier, firmer and less rich.
• Whipping cream instead of double cream. It works but is a little lighter and less stable, so whip it carefully and keep the cake cold.
• Ground pistachios instead of paste. This is not an equal swap. Ground nuts are drier and slightly gritty, and the cream will not be as smooth (try and weigh).

Storage

Covered in the fridge for about 2 days (our estimate, since it depends on the freshness of the cream). If it stiffens, stir it gently with a spatula instead of whipping it again.