Preheat the oven to 250° and position a rack in the lower third. Line a baking sheet with parchment. In a bowl, beat the egg whites with the cream of tartar at medium speed until frothy. Beat in 3/4 cup of the sugar, 1 tablespoon at a time, until the meringue is thick and glossy; scoop into four 4 1/2-inch mounds onto the prepared sheet. Using the back of a spoon, make a well in the center of each mound; bake for 1 hour, until crisp but still chewy inside. Turn off the oven and prop the door open; let cool for 1 hour.