1. Fill a medium saucepan with 2 inches/5 cm of water and bring to a low boil.
2. Remove the zest from the oranges with a rasp-style grater and place in a stainless steel or tempered glass bowl that fits snugly into the heating saucepan. Cut the oranges in half and juice them until you have 1 1/4 cups/300 ml of orange juice. Add the orange juice, sugar, cardamom, and egg yolks to the zest and whisk. Once the mixture is mixed well, set the bowl on top of the saucepan. Switch to a silicone spatula and stir continually as the orange curd begins to cook.
3. As you stir, monitor the temperature of the curd with a candy or instant-read thermometer. The curd will begin to thicken between 190 and 195 degrees F/about 90 degrees C. Once it looks thick in the bowl and coats the back of a teaspoon, it is done. You don't want to let it cook beyond 205 degrees F/95 degrees C, as higher temperatures can cause it to curdle. Over medium-high heat, this curd typically takes 14-18 minutes to thicken.
4. When the curd has thickened, drop in the butter and stir until melted. Once the butter is fully incorporated, remove the curd from the heat.
5. Strain the curd through a fine-mesh sieve into a bowl. The straining removes both the zest (which will have imparted a great deal of flavor to the curd during the cooking time) and any bits of scrambled egg.
6. Pour the strained curd into 2 half-pint/250 ml jars for storage. When it has cooled to room temperature, store in the refrigerator or freezer. It will keep for up to 10 days in the fridge and up to 6 months if frozen.