Pour heavy cream into a saucepan and heat over medium-high. Using a candy thermometer to check temperature, cook until it reads about 190°F, about 5 minutes. Stir with a silicone spatula while cooking to ensure the cream doesn’t scorch on the bottom of the pan.
Stir in the lemon juice and continue to simmer at the same temperature, stirring continuously, for another 5 minutes. The mixture will thicken quickly and begin to bubble slightly, so reduce heat as needed.
Remove from heat and let rest at room temperature for 30 minutes.
Line a strainer with cheesecloth and set over a bowl. Pour the cooled mixture into the cheesecloth, making sure the cheesecloth stays in place. Cover the strainer with plastic wrap and let drain in the refrigerator for 8 hours, or overnight.
After resting, put the finished cheese in a bowl and stir to loosen it up. Stir back in some of the whey that drained out if you like, to achieve the texture you want.
Any other whey can be saved and used in smoothies, batters, or scrambled eggs.
Store mascarpone cheese in the refrigerator for up to a week.