In a large pot, bring the peeled and halved russet potatoes to a boil in salted water.
Reduce the heat to medium-low and cook them until they are very tender, which should take about 15 to 20 minutes.
Drain the potatoes in a colander and let them drain thoroughly for about 5 minutes.
Preheat your oven to 425°F (220°C).
In a small bowl, stir together the mascarpone and egg yolk with a wooden spoon or spatula until it becomes smooth.
Stir in the milk and whisk until the mixture is smooth.
Season it with salt, black pepper, and cayenne pepper.
Transfer the cooked potatoes to a very large bowl.
Add the chunks of butter and season with additional salt and black pepper, if desired.
Mash the potatoes with a potato masher until they become fluffy.
Then, stir in the mascarpone mixture.
Spread the mashed potatoes into a 9×13-inch (3-quart) baking dish.
Smooth the top, and use the tip of a spatula to lightly press and lift the potatoes, creating small peaks and valleys for browning.
(If you plan to make this dish ahead of time, don’t preheat the oven until you’re ready to bake.
Assemble the dish through Step 4, then cover it with foil and refrigerate for up to 1 day.
Bake, covered, for 20 minutes, then uncover and continue to Step 5.
)
Bake the potatoes until they are heated through and the top is golden brown, which usually takes about 20 to 25 minutes.
If desired, garnish with sliced green onions and red pepper flakes.
Enjoy your rich and creamy Mascarpone Mashed Potatoes!
