Put a small roasting pan filled halfway with hot water in bottom third of oven (to provide moisture during baking), then preheat oven to 325 degrees F. Butter a 10-inch springform pan and line bottom with a round of parchment or waxed paper. Butter the paper.
Melt butter and chocolate together in a heavy saucepan over low heat, stirring, then remove from heat. Beat together yolks, 1/3 cup sugar, and 1/2 teaspoon salt in a large bowl with an electric mixer until thick and pale, and ribbons form when beater is lifted, about 6 minutes. Beat whites at medium speed with cleaned beaters in another large bowl until they just hold soft peaks. Gradually add 1/3 cup sugar, beating until whites just hold stiff peaks. Stir warm chocolate mixture into yolk mixture until combined well. Stir 1/4 of egg whites into chocolate mixture to lighten, then fold in remaining egg whites gently but thoroughly.
Pour batter into springform pan and bake in middle of oven (do not place springform pan in pan of hot water) until a tester inserted in center comes out with crumbs adhering, about 1 hour (a crust will form and crack on top of cake as it bakes). Transfer to a rack and cool 10 minutes (cake will "deflate" as it cools). Run a thin knife carefully around edge of cake, then remove side of pan. Cool cake on bottom of pan 30 minutes, then invert onto another rack or plate. Remove bottom of pan, then carefully peel off parchment. Invert cake onto a serving plate.
Meanwhile, remove zest from 2 oranges and trim any white pith from zest. Cut zest into enough very thin strips to measure 1/4 cup. Squeeze juice from 3 oranges and strain through a fine sieve. Measure out 1 cup juice.
Cook remaining 1/2 cup sugar in a dry heavy saucepan over moderate heat, undisturbed, until it begins to melt. Continue to cook, stirring occasionally with a fork, until sugar is melted into a deep golden caramel. Add zest and cook, stirring, until fragrant, about 15 seconds. Tilt pan and carefully pour in juice (caramel will harden and steam vigorously). Cook over moderately low heat, stirring, until caramel is dissolved, then cool sauce.
Serve cake with orange sauce.
Cook's Note: We recommend making this cake 1 day ahead to allow flavors to develop. Keep chilled, covered with plastic wrap. Bring to room temperature before serving.