Position racks in the upper and lower thirds of the oven; preheat to 250 degrees F. Line 2 baking sheets with parchment paper.
Beat the egg whites, cream of tartar and salt in a large clean dry bowl with a mixer on medium speed until foamy. Increase the speed to medium high and beat until thick and opaque, about 1 more minute. Gradually add the confectioners' sugar, about 1/4 cup at a time, and beat until stiff shiny peaks form, about 5 more minutes.
Transfer the meringue to a pastry bag fitted with a large round tip. Pipe eight 3-inch disks on the baking sheets, then pipe around the edge of the disks to create 2-inch-high sides. Bake at 250 degrees F until dry and crisp, about 3 hours. Let cool completely on the baking sheets. Store in an airtight container in a dry place for up to 1 week.
For the filling, whisk the mascarpone with the confectioners' sugar, then whisk in the heavy cream until soft peaks form. Spoon into the meringue nests; top with sugar pearls.