Wash four or five glass jars and their lids in hot soapy water, rinse thoroughly, and place them upright on a baking tray. Heat in the oven at 120°C (250°F) for fifteen minutes. This sterilizes the jars and is a non-negotiable step for safe, long-lasting apricot jam. Place two small plates in the freezer now for the set test later.
Wash, halve, and pit the apricots. There is no need to peel them — the skins will dissolve during cooking. If you wish to include blanched kernels, crack open a few apricot stones with a nutcracker, remove the kernels, blanch in boiling water for two minutes, drain, and set aside. Cut any particularly large apricot halves into quarters for more even cooking.
Combine the prepared apricots with the sugar, lemon juice, and zest in a large, heavy-bottomed pot. Stir to combine, then cover and leave to macerate at room temperature for a minimum of one hour — or overnight in the refrigerator. This draws out the fruit's natural juices, dissolves much of the sugar before heat is applied, and results in a fresher, more vibrant jam.
Place the pot over medium heat and stir gently until all the sugar has fully dissolved — this takes approximately five to eight minutes. Do not allow the mixture to boil before the sugar has dissolved completely, as undissolved sugar crystals can cause the finished jam to crystallize during storage. Once the sugar is fully dissolved, increase the heat to medium-high and bring the jam to a rolling boil.
Once at a boil, add the butter, vanilla pod, cardamom (if using), and the blanched apricot kernels. Stir to incorporate. The butter will immediately begin to reduce the surface foam. Skim off any remaining foam with a spoon — a clean jam is a sign of careful technique and prolongs shelf life.
Maintain a vigorous boil and stir regularly to prevent the jam from catching on the bottom of the pot. If you have a sugar thermometer, clip it to the side of the pot and cook until the jam reaches 104–105°C (219–221°F). This typically takes between twenty and thirty-five minutes depending on your stove and the water content of the fruit. If using the cold plate test, begin testing after twenty minutes of boiling.
Remove the vanilla pod and any apricot kernels. Take the sterilized jars from the oven using a clean cloth. Ladle the hot apricot jam carefully into the jars, filling to within half a centimeter of the top. Wipe the rims clean with a damp cloth, seal the lids firmly, and turn the jars upside down for five minutes — this creates a vacuum seal as they cool. Turn right-side up and leave to cool completely, undisturbed, on a wire rack.
Once fully cooled, check that the lids have sealed (they should be concave and not flex when pressed). Label each jar with the date. Store in a cool, dark place. A properly sealed jar of apricot jam will keep for up to twelve months. Refrigerate after opening and use within four to six weeks.
