Preheat oven to 350 degrees F (175 degrees C).

Beat confectioners' sugar and peanut butter together in a bowl with an electric mixer until smooth. Spread peanut butter mixture into prepared pie crust as evenly as possible.

Whisk water, cornstarch, and 1 tablespoon white sugar together in a saucepan; bring to a boil for 15 seconds, stirring constantly. Remove saucepan from heat and cover with a lid.

Beat egg whites in a glass or metal bowl until soft peaks form. Lift the beater or whisk straight up: the egg whites will form soft mounds rather than a sharp peak.

Beat cream of tartar and vanilla into egg whites. Gradually add 1/2 cup white sugar, continuing to beat until stiff peaks form. Lift the beater or whisk straight up: the egg whites will form sharp peaks.

Beat cornstarch mixture into egg white mixture on lowest speed, 1 tablespoon at a time. Increase speed to medium and beat egg whites for 10 seconds to form meringue. Spread meringue over peanut butter mixture to the edges of the pie crust.

Bake in the preheated oven until meringue peaks are golden, 10 to 15 minutes.