In a large bowl, with a mixer on high speed, beat egg whites and cream of tartar until frothy. Continuing to beat at high speed, add sugar in a slow, steady stream; continue to beat until mixture holds stiff peaks, 8 to 12 minutes. Beat in vanilla.
Scrape into a buttered and floured 8-inch cake pan with a removable rim at least 3 inches tall. With a flexible spatula, spread mixture out to edges of pan and smooth out top.
Bake in a 300° regular or convection oven until top is lightly browned and feels firm when gently pressed in the center, about 25 minutes.
Remove from oven and let stand for 20 minutes (meringue may shrink a little as it cools). Run a thin knife between meringue and pan rim, then remove rim. Carefully invert meringue onto a plate, then slide a long, thin spatula between pan bottom and meringue and gently lift the bottom off. Invert meringue again onto another plate so that it is right side up. Let cool completely.
In a bowl, whisk hazelnut liqueur into chocolate-hazelnut spread. In another bowl, whip cream until it holds soft peaks. Stir about 1/4 cup of the whipped cream into the chocolate mixture until no white streaks remain. Fold chocolate mixture into remaining whipped cream, then whisk gently until mixture has a loose, spreadable consistency (take care not to overbeat or it will curdle).
With the spatula, spread chocolate-hazelnut cream around sides and over the top of the meringue. Chill torte uncovered for at least 1 hour, or cover with a large, inverted bowl (it shouldn't touch cream) and chill up to 24 hours.
Cut torte into wedges to serve.