Preheat the oven to 180 C. Line an 8" skillet or round cake tin with greased baking paper. The cake is sticky, so I highly recommend using paper for easy removal and slicing. If you don't have it, make sure you grease the skillet or tin very well.
Melt the butter, making sure it doesn't get boiling hot. Set aside.
In a mixing bowl, combine the eggs and sugar and whisk very well until pale and slightly thickened, about 1 minute. Use a balloon whisk here for the best result, or a hand mixer on low speed. Stir in the vanilla extract.
Now sift in the flour, cocoa and salt (if using). Gently fold it all together to combine using a silicone spatula preferably. Finally, pour in the melted butter and stir to get a smooth, not super thick batter.
Pour into the prepared tin and bake for 25 to 30 minutes until the surface no longer looks wobbly or liquidy, but is still soft to touch. Don't over-bake the cake or it'll harden too much as it cools.
Let the cake cool for about an hour at room temperature. The surface will sink and feel crusty. Before diving in, sift the top with 2 tbsps cocoa powder. When you slice it, the inside will be fudgy and gooey! The longer it rests, the more the texture and flavour improves.
This cake can be stored at room temperature for a day or two. Since it is so soft and gooey, I prefer storing it in the fridge after day one, where it will keep for about a week (the cocoa dusted on top will become a bit wet). Gently reheat as needed. Happy baking!