*This will make more lemon curd than you need but it's difficult to make a smaller amount without over cooking the egg yolks.
In a large metal bowl or the top portion of a double boiler, add in the egg yolks, sugar, salt, lemon zest and lemon juice. Whisk to combine.
Add about 2″ of water into a sauce pan or the bottom of the double boiler. Bring to a simmer and then reduce the heat to low so that it maintains a simmer.
Place the bowl with the egg and lemon mixture over the pot of simmering water. Whisk constantly, cooking the mixture until it thickens. It’s done when the mixture is thick like mayo, essentially when you stop whisking the mixture should stop moving, it will sort of just firm up.
Once thick, immediately remove the bowl from the pot of water and turn off the heat.
Add the cold mascarpone into the hot curd, one spoonful at a time. Whisk one spoonful of mascarpone into the lemon curd until completely dissolved before adding in the next spoonful.
Continue adding the mascarpone in until it is all used up.
Pour the mixture through a sieve into a large shallow bowl and place a piece of plastic onto the surface. Place the bowl in the fridge until cold and thick.