500ml whipping cream, 36%
1 tbsp. lemon juice
Create a double boiler with a pot and a metal boil. Fill the pot with enough water so that it doesn't touch the base of the bowl when you put the bowl over top of the pot.
Boil water and reduce to medium heat. Put the bowl on the pot and add whipping cream.
Stir occasionally until the cream reaches 190 degrees fahrenheit, or until it bubbles. It should take 10 to 15 minutes. Once it bubbles add lemon juice.
Gently stir in the lemon juice. Let it stand for a few minutes and whisk occasionally. Do this until it begins to thicken. This can take up to another 15 minutes.
Once its thick enough remove from heat. Pour mixture into a layer of 3 coffee filters and place in a colander overtop a pot. Place in the fridge overnight, and through the next day if necessary.
The result should be a dense, spreadable cream. In essence, mascarpone.