Soften the butter on medium heat until half-way melted.
Remove the half-melted butter from the heat, and continue stirring until fully melted.
Whisk in the sugar, salt, eggs, vanilla extract, and cocoa powder. Stir until fully incorporated.
Slowly add in the flour, a little at a time, until smooth.
Transfer the batter to a greased, parchment-lined, 9" spring-form pan that you have dusted with semolina flour to prevent sticking.
Bake the cake in a 300°F or 150°C oven for 22 minutes. It is better to under-bake than over-bake this cake. For a molten cake, allow to cool for about 10 minutes before adding berries, whipped cream, ice cream, and/or powdered sugar and serving. For a firmer, stickier cake, allow to cool completely before garnishing and serving.
