Preheat oven to 350 degrees. Lightly butter springform pan and dust with sugar, tapping out any excess.
In a large heatproof bowl, combine chocolate, oil and stick of butter, and set over pan of simmering water and heat, stirring often, until melted. When melted, remove bowl from saucepan and let rest.
Separate 4 eggs, placing the whites and yolks in separate medium bowls. To the yolks, add the cocoa powder, vanilla, salt and 1/4 cup of sugar and remaining 2 eggs. Whisk mixture until it is smooth. Gradually whisk the yolk mixture into the chocolate mixture, blending well.
Using an electric mixer or stand-up mixture on high speed, beat the egg whites until they are frothy. With the mixer running carefully and slowly add the sugar, beating until soft peaks form.
Now, gently fold in the egg whites into the chocolate--in two batches. Fold just until incorporated.
Scrape the batter into the prepared springform pan, smooth the top and sprinkle with the remaining 2 tablespoons of sugar.
Bake until top is puffed and starting to crack and the cake is pulling away from the sides of the pan, about 35-45 minutes. Transfer to a wire rack and let cake cool completely in pan. The cake will deflate in the center and crack further as it cools.
For the topping, use an electric or stand-up mixer on medium-high speed to beat the cream, mascarpone and powdered sugar until soft peaks form.
To dazzle your guests, remove the cake from the springform pan and mound whipped cream mixture in the center of the cake. Alternatively, if it's just two of you, place a piece of cake on a plate and generously place a mound of cream on top so it is spilling over.
Refrigerate any leftover cake and cream.
If, after a few days, any cake remains, just whip up a fresh batch of mascarpone cream and make individual trifles. Let the layers of chopped bites of cake and cream melt together in the fridge for a few hours before serving.