In a heatproof bowl over a saucepan of simmering water, place the egg whites, sugar and cream of tartar. Whisk constantly until sugar is dissolved and egg whites are warm, about 5 minutes.
Whip mixture with the whisk attachment of a stand mixer until the mixture is lofty, glossy and holds stiff peaks. Add vanilla, mix well and dollop mixture all over the top of the pie, aiming for about 6 inches of height. Brulee the meringue with a torch. Serve pie immediately at room temperature.