Preheat oven to 350°F. Grease 9-inch springform pan; line bottom and sides with parchment paper. Set aside.
In heatproof bowl set over saucepan of simmering (not boiling) water, melt chocolate. Stir in butter and remove from heat. Let cool slightly.
In bowl of stand mixer fitted with whisk attachment, beat egg yolks and half of the sugar on low until combined. Increase speed to medium-high, and beat until mixture is pale, thick and doubled in volume, about 3 to 5 minutes.
Using a rubber spatula, stir in cooled chocolate mixture until combined, scraping bottom and sides of bowl. Add cornstarch and salt, folding to combine; scrape into large bowl. Wash and dry bowl and whisk attachment of stand mixer.
In bowl of stand mixer fitted with whisk attachment, beat egg whites on low until foamy; gradually add remaining sugar, then increase to medium-high and beat until firm peaks form.
Fold one-third of the meringue into chocolate until a few streaks remain. In two stages, gently fold in remaining meringue until no streaks remain. Pour batter into prepared pan, and gently level with spatula.
Bake until centre of cake is just set and crust has formed on top, about 23 to 28 minutes; centre of cake should still be moist. Let cool completely in pan on rack; the cake will sink and may crack during cooling. Remove collar.
To serve, dust icing sugar over cake using a fine sieve.