Whisk together 1 1/4 cups sugar, lemon juice, 3 egg yolks (reserving whites), cornstarch, and 1 1/4 cups water in a medium saucepan; cook over medium heat, whisking constantly. When mixture begins to thicken and bubble (about 10 minutes), remove from heat and stir in butter. Transfer custard to 4 (6-ounce) ovenproof baking cups. Place plastic wrap directly on surface of custard; set aside.
Preheat oven to 400°. Bring water to a boil in a large pot (into which a large glass bowl can be nestled), and have a candy thermometer ready. Combine egg whites, remaining sugar, salt, cream of tartar, and vanilla in the glass bowl, and place it over pot with boiling water; immediately start beating egg white mixture with a handheld mixer until temperature reaches 160°, about 5 minutes. Remove bowl from heat and continue to beat until meringue forms stiff peaks. Remove plastic wrap from custard and top each with meringue, sealing meringue to the cup's edges. Place cups on a sheet tray; bake until meringue is golden brown, 6 to 8 minutes, rotating tray after 4 minutes to brown evenly. Remove from oven, and cool on a rack. Serve at room temperature or chilled.Note:
For an extra treat, zest 2 lemons before juicing to create a lemon-infused sugar