Fallen Chocolate Cake

Preheat oven to 350 °F. Lightly butter springform pan and dust with sugar, tapping out any excess. Combine chocolate, oil, and 1/2 cup butter in a large heatproof bowl. Set over a saucepan of simmering water and heat, stirring often, until melted. Remove bowl from saucepan. Separate 4 eggs, placing whites and yolks in separate medium bowls. Add cocoa powder, vanilla, salt, 1/4 cup sugar, and remaining 2 eggs to bowl with yolks and whisk until mixture is smooth. Gradually whisk yolk mixture into chocolate mixture, blending well. Using an electric mixer on high speed, beat egg whites until frothy. With mixer running, gradually beat in 1/2 cup sugar; beat until firm peaks form. Gently fold egg whites into chocolate mixture in 2 additions, folding just until incorporated between additions. Scrape batter into prepared pan; smooth top and sprinkle with remaining 2 tablespoons sugar. Bake until top is puffed and starting to crack and cake is pulling away from edge of pan, 35-45 minutes. Transfer to a wire rack and let cake cool completely in pan (cake will collapse in the center and crack further as it cools). DO AHEAD: Cake can be made 1 day ahead. Cover in pan and store airtight at room temperature.