If you are making your own apple juice, start by juicing the apples and keep the liquid only.
Place the apple juice, egg yolks, sugar, lemon juice and cinnamon in a non-reactive, heavy based medium saucepan. Whisk until combined.
Turn on low heat and cook for 3 to 4 minutes, continuously stirring with a heat-proof spatula or wooden spoon. The sugar should have completely dissolved and the apple mixture should start to thicken very slightly (see note 1).
Add the cubed butter a little bit at a time, stirring until completely melted in the mixture before adding more. Once all the butter has been mixed in, continue to cook in the pot for another 5 to 8 minutes (more or less depending on the heat and size of saucepan and heat of the stove). The curd should have thickened and coat the back of a spoon or spatula (see note 2).
Optionally, pour the cooked apple curd through a fine mesh sieve to discard any lumps or bits of cooked eggs.
Let the curd cool down slightly then transfer inside glass jars with lids or in an airtight container, covering the top of the curd with plastic wrap touching its surface. Place in the fridge to chill for at least 2 hours, preferably overnight. The curd will continue to set as it chills. Keep in the fridge.
Notes
Make sure not to keep the temperature on very low or the eggs will cook too quickly and start to scramble, resulting in a curdle or lumpy curd. The curd should only simmer and never boil.
Ideally, use a kitchen thermometer to check if the curd is cooked; it is cooked when it reaches about 82 degrees Celsius (180 degrees Fahrenheit).