Put the cream in a saucepan and heat to about 85° C (175° F) until simmering. Make sure the temperature stays constant and check with the kitchen thermometer.
Stir the lemon juice into the cream, which will start to curdle. Allow the mixture to thicken for a further 10 minutes while stirring, then remove the saucepan from the heat.
Let the curds and whey cool for about 15 minutes and rest for 1 hour.
Place a sieve on top of a bowl and line it with the cheesecloth. Pour the mixture into the sieve and let the whey drain in the fridge, covered with a kitchen towel, for about 12 hours.
Now you can enjoy the homemade mascarpone immediately afterwards or transfer it to a sterilised, sealable glass jar.