Pour cream into a heavy bottom saucepan over medium heat.
Clip a candy thermometer to the pot, being sure that the probe touches the cream. Cook, stirring frequently to prevent scorching, until the cream reaches 190°F.
Add the lemon juice to the pot and stir to combine. Continue cooking, stirring constantly with a rubber spatula, for another 5 minutes, maintaining the temperature at 190°F. The mixture will thicken.
Take the pot of mascarpone off the heat and allow it to cool to room temperature.
Line a fine mesh sieve with multiple layers of cheesecloth and place it over a bowl. Transfer the thickened cream into the lined sieve and refrigerate it, with the bowl underneath, for at least 8 hours.
Use a spatula to scrape the mascarpone from the cheesecloth into an airtight container. Cover and store in the refrigerator for up to 1 week.
Notes
Use pasteurized heavy whipping cream, but not ultra-pasteurized.Squeeze the lemon juice through a strainer or into a separate container and then measure out a tablespoon. This way, you will not drop seeds in the mascarpone.Making mascarpone is very similar to making whole milk ricotta. However, the higher fat content in cream for mascarpone prevents it from curdling/separating as it does for ricotta.