Preheat the oven to 350℉/180℃. Lightly grease the bottom of a 6-inch/15 cm springform pan with margarine or vegetable oil. Line the bottom with parchment paper. brush the bottom and the sides of the pan with upward motions. Lightly coat the inside of the pan with sugar, rotating the pan to coat the entire surface, then tap out the excess. Set aside.
In a large bowl, add melted chocolate, vegetable oil, almond flour, and egg yolks. Stir to combine. Set aside.
In a stand mixer fitted with the whisk attachment or in a large bowl if using a hand mixer, beat egg whites and salt on low speed until foamy. Gradually add castor sugar little by little and increase the speed to medium-high speed. Beat until stiff peaks form.
Transfer one-third of the egg white mixture to the chocolate mixture. Fold with a spatula until well combined. Add the remaining egg white mixture in 2 additions, gently folding it into the chocolate mixture each time.
Pour the cake batter into the pan and bake for 25 minutes or until a toothpick inserted into the center comes out with a few moist crumbs. Don't overbake or you will end up with dry cake. Remove from the oven and let the cake cool completely in the pan. It will gradually fall and slump in places, which is fine.