Start adding the heavy cream to a saucepan. Heat the cream to 82-85 °C (180-185 °F). Use a thermometer to monitor the temperature, stirring occasionally.
Meanwhile, squeeze the lemon and strain the juice. Measure 7.5 ml of lemon juice with a syringe.
Once the heavy cream reaches 82 °C (180 °F), turn off the heat and continue mixing for 3–4 minutes as the temperature decreases to 60 °C (140 °F).
Return the saucepan to heat and raise the temperature back to 82-85 °C (180-185 °F) to further thicken the cream.
Once 82-85 °C is reached again, add the lemon juice or citric acid and mix well for 1–2 minutes until thickened.
Remove from heat while continuing to stir and allow to cool slowly to 55-60 °C (130-140 °F).
Transfer the cream to a glass bowl and cover it with a lid that is not completely airtight. Cover with a cloth and let sit at room temperature for 3 hours. Check after the required time that the cream has thickened like yogurt.
To further thicken, place a clean cloth over another glass container and use tweezers to keep it elevated. Carefully add the mixture, avoiding contact with the bottom to allow the mascarpone to drip and separate the fat from the liquid. Cover and refrigerate for 24 hours.
The next day, the mascarpone will be ready. Transfer to an airtight glass or ceramic container. It can be enjoyed immediately or stored covered in the refrigerator for up to 3 days.
The mascarpone can now be used to prepare tiramisu or other desserts requiring this ingredient!