To make the traditional Italian recipe using fresh egg yolks
In a small bowl, whisk 1egg yolk(s) together with the sugar until frothy. In a separate bowl, stir the mascarpone with a fork to loosen it up. Add the rum (if using) and blend. Gently incorporateabout a third of the whisked yolk(s) into the mascarpone at a time. If too thick, add another yolk, until desired consistency.
Variation - with cooked egg yolks
Fill a saucepan with about 1 inch (2.5cm) of water, and bring to a simmer. Meanwhile, in a heatproof bowl, whisk the egg yolks with the sugar until frothy and pale yellow. Place the bowl over the saucepan, making sure it's not touching the simmering water. Add the rum (if using) and whisk for about 6-8 minutes, or until the mixture thickens like a custard and reaches 140°F(60°C). Let cool at room temperature for about 20 minutes, then blend into the mascarpone cheese. Avoid blending with the mascarpone while the egg mixture is still hot, as it will curdle.
Variation - without eggs
Remove the mascarpone from the refrigerator 15 minutes before starting. Stir it with a fork to loosen it up. Whisk in the sugar. Then whisk in the rum (if using).
Ingredient Notes
Mascarpone - Get the good stuff to ensure it is thick (Or you can easily make your own. See mascarpone recipe.)
Eggs - How many eggs you'll need depends on the size and quality of the eggs, and the consistency of the mascarpone cheese. You want it as thick as possible, especially if adding any liquid flavoring.
Sugar - Sift for extra smoothness. For layered desserts like tiramisu, 3 Tbsp of sugar is enough. For serving as a dessert on its own, I prefer to use 4 Tbsp.
Recommended Flavoring Variations
Alcohol - Marsala, limoncello, Grand Marnier (orange Cognac brandy).
Coffee - cooled strong espresso (2 scant Tbsp or less, to preserve thickness), either blended in or poured over. For a stronger coffee flavor while preserving the cream's thickness, try coffee extract.
Vanilla - either 1 tsp real vanilla extract or the seeds scraped out of ½ vanilla bean.
Cinnamon - Blend in to your taste and slather on cinnamon rolls.
Chocolate shavings (Use a vegetable peeler on a dark chocolate bar.) Stir in a little or a lot.
Recommended Serving Ideas
Layer into - tiramisu or a fruit bowl or cups.
Top with - Fabbri amarena (dark cherries) in syrup, chocolate shavings, or berry compote (see recipe).
Slather on - french toast, cinnamon rolls, slices of cake, or one of the traditional Italian holiday cake-like breads such as pandoro or panetone.
Storing
Refrigerator - Store covered in fridge. Do not store at room temperature. The consistency is best if eaten within 1 to 2 days, and is necessary with fresh egg yolks.
Freezer - Not ideal, but doable. Cover tightly. Thaw slowly in the refrigerator, stirring occasionally with a fork.