Step 1

Fill a large pot with salted water and bring to a boil over high heat. Meanwhile, place potatoes in a small pot filled with salted water. Set pot over high heat and bring to a boil. Cook until potatoes are fork-tender, about 15 minutes. Strain and toss dry.

Step 2

When potatoes are cool enough to handle, peel. Working over a bowl, pass warm potatoes through a ricer or grate on medium holes of a box grater. Stir in mascarpone. Break up any clumps so mixture is completely smooth. Grate fontina on medium holes of box grater and stir into potato mixture along with 1 whole egg until fully combined. Season with salt.

Step 3

Cut sheets of pasta into triangles so short sides are each about 3 inches long. Brush outer edges with beaten egg. Spoon about 1 tablespoon potato mixture into center of half the pasta triangles. Top stuffed triangles with remaining triangles. Use the tines of a fork to crimp edges together to completely seal.

Step 4

Working in batches to avoid crowding, carefully drop stuffed pasta into large pot of boiling water. Once each triangle floats up and bobs at water’s surface for about 1 minute, use a slotted spoon to fish it out and transfer to a sheet pan slicked with olive oil.

Step 5

While pasta boils, melt butter in a medium pan over medium-low heat. Once butter turns golden brown and smells nutty, after 3-5 minutes, remove pan from heat.

Step 6

Arrange warm pasta on 4 plates and pour browned butter overtop. Top with finely grated Parmesan and serve immediately.