Place egg whites in a large bowl; let stand at room temperature for 30 minutes. Add vanilla and cream of tartar; beat on medium speed until soft peaks form. Gradually add 1 cup sugar, about 2 tablespoons at a time, beating on high until stiff and glossy peaks form and sugar is dissolved.
Spread in a well-greased 13x9-in. baking dish. Bake at 300° for 1 hour. Cool on a wire rack.
Meanwhile, in the top of a double boiler, whisk egg yolks until lemon-colored. Add the lemon juice, zest and remaining sugar. Cook over hot but not boiling water, stirring occasionally, until thickened and a thermometer reads 160°. Cool to room temperature.
Spread half of whipped cream over meringue shell; cover with lemon mixture. Top with remaining whipped cream. Cover and refrigerate. Best made one day before serving.