In a small saucepan, melt dark chocolate, milk, and oil together on low heat until smooth. Set aside to cool slightly.

Sift together cocoa powder, flour, and cornstarch into a bowl. Add vanilla extract and egg yolks, whisking until smooth. Stir in the melted chocolate mixture.

In a separate clean bowl, beat egg whites with salt until foamy. Gradually add sugar in 2–3 additions while beating. Continue until stiff peaks form.

Whisk ⅓ of the meringue into the chocolate batter to loosen it. Then gently fold the batter into the remaining meringue using a spatula, keeping as much air as possible.

Line a 5-inch round cake pan with parchment (leaving overhang). If using a springform or removable-bottom pan, wrap it in 2–3 layers of foil to prevent leaks. Pour in the batter. Place the cake pan into a roasting pan and fill with 2–3 inches of hot water. Bake at 150°C (302°F) for 45–50 minutes, until a skewer comes out clean.

In the last 10 minutes, sprinkle chopped chocolate or chips (optional).

Remove the cake from the pan with the help of the parchment overhang while still warm. Serve warm with powdered sugar or chilled with melted chocolate.