Soufflé Cake: Put a small roasting tray filled half way with hot water in bottom third of oven to provide moisture during baking. Position another oven rack in middle of oven then preheat oven to 180 degrees C. Butter springform cake tin and line bottom with a round of baking paper. Butter 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 salt in a large bowl with an electric mixer at medium-high speed until thick enough to form a ribbon that takes 2 seconds to dissolve into mixture when beater is lifted; 6 to 8 minutes in a stand mixer or 10 to 14 minutes with a handheld.
Stir warm chocolate mixture into yolk mixture until combined well.
Whisk whites in another large bowl with cleaned beaters at medium speed until they just hold soft peaks.
Gradually add 1/3 cup sugar, beating until whites just hold stiff peaks. Stir one fourth of whites into chocolate mixture to lighten then fold in remaining whites gently but thoroughly.
Pour batter into springform cake tin and bake in middle of oven (do not place springform cake tin in hot water) until a wooden skewer inserted in centre comes out with crumbs adhering; about 1 hour (a crust will form and crack on top of cake as it bakes).
Transfer to a cake rack and cool 10 minutes; the 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. Place a rack over cake and invert cake onto rack then remove bottom of pan and carefully peel off paper. Place a serving plate over cake then invert cake onto plate.
Orange Caramel Sauce: Remove zest from 2 oranges and trim any white pith from zest with a paring knife. Cut enough very thin strips of zest to measure 1/4 cup.
Squeeze juice from all 3 oranges and pour through a fine-mesh sieve into a bowl. Measure out 1 cup juice and reserve remainder for another use.
Cook remaining 1/2 cup sugar in a dry heavy based 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 saucepan and carefully pour in juice, caramel will harden and steam vigorously. Cook over moderately low heat stirring until caramel is dissolved then cool sauce.
To Serve: Ideally the cake should be made the day before to allow the flavours to develop. Bring cake to room temperature then serve with the orange caramel sauce.