Orange curd is a soft, glossy citrus filling that adds moisture and a sharp contrast to plain sponge and buttercream. It goes wrong in two ways: it scrambles into tiny lumps, or it tastes flat and merely sweet. The first happens when the eggs get too hot, the second because orange juice is much milder than lemon.

Ingredients and what each one does

Oranges. The juice is the liquid and the flavour, and the zest holds the aromatic oils. Most of the “orange” smell is in the zest, not the juice.
Caster sugar. It sweetens and also delays egg coagulation, so you have a wider safe window before the curd scrambles. Fine crystals dissolve fast and also rub the oil out of the zest.
Butter. It adds gloss, smoothness and a soft set once cold. It goes in at the end.
Eggs or yolks. They are the thickener, and the 80–82 °C target below comes from them. Our ingredient table lists the oranges, sugar and butter, so add eggs or yolks if you want a real curd. Without them you only get a reduced orange-butter sauce, which will not hold a 0.6 cm layer. How many to use is a matter of taste: more yolk gives a denser, richer curd (try and weigh).

How to make it

Zest the oranges first and rub the zest into the sugar with your fingertips. The sugar crystals scrape open the oil cells, and the oil dissolves into the sugar. Sign: the sugar looks damp and smells strongly of orange. This takes 1–2 minutes.
Squeeze the juice. Optional: simmer it gently until it reduces, because evaporating water concentrates the flavour. Sign: the volume is clearly lower. Reducing by about a third is our estimate, so try and weigh. Cool the juice fully before the eggs, because hot juice cooks them on contact.
Whisk the eggs with the zesty sugar and add the juice. The sugar coats the proteins and raises the temperature at which they set.
Cook over low heat, stirring constantly, to 80–82 °C. Egg proteins unfold and link up into a net that traps the liquid, and above about 85 °C that net tightens and squeezes the water out as lumps. Sign: the curd coats the back of a spoon and a finger-drawn line stays open. This takes roughly 5–12 minutes. A double boiler is safer but not required; direct low heat works if you stir the whole time and use a thermometer.
Strain at once through a fine sieve into a clean bowl. The sieve removes any specks of overset egg, and pouring it out of the hot pan stops the cooking. Sign: smooth, glossy and uniform. Leaving the zest in or straining it out is a matter of taste, since the flavour is already in the curd.
Add the cold butter in cubes off the heat and stir or blend until melted. The butter emulsifies into the warm base and cools it, which gives the gloss. Sign: no oily streaks.
Press cling film directly onto the surface and chill. This stops a skin forming, and the fat firms as it cools, so the curd thickens further. Sign: it holds a spoon trail and does not run. This takes roughly 4–6 hours.

What can go wrong

Tiny lumps. The eggs went past about 85 °C, or the heat was too high. Strain, or blend with a stick blender. Do not add anything to “fix” it.
Too thin. Either it never reached 80 °C, or the eggs were too few. Return it to low heat and bring it to 80–82 °C. Remember that it thickens a lot more once cold.
Flat, just sweet. Orange juice has less acid than lemon. A small splash of lemon juice sharpens it, which is a matter of taste (try and weigh).
Metallic taste. The acid reacted with an aluminium pan. This is a general rule: use stainless steel or enamel.
Oily or split. The butter went into a base that was too hot, or too much was added. Cool the base a little first, then blend.

Substitutions

Blood orange or mandarin juice works the same way, but check the sweetness and acidity, as they vary.
Margarine instead of butter is not recommended: it has less fat and more water, so the curd will be looser and duller.
A little cornflour lets you boil the curd safely, but the texture is less silky. Try and weigh it against the plain version.

Storage

In a closed jar in the fridge for up to 5–7 days, with cling film on the surface once opened. Stir it briefly before using. In a cake, pipe a cream dam around the edge, because this soft filling will otherwise squeeze out under the weight of the layers.