Preheat the oven to 200°C. In a small saucepan, melt the butter and set aside.
In a bowl, combine sugar, eggs, vanilla and salt and mix for a couple of minutes at low speed. Do not overmix - keep the speed low and stop when the ingredients are combined.
Once combined, add the flour and cocoa powder and start the mixer again, always at low speed. As you are mixing, pour in the melted butter and continue mixing. Stop when everything is evenly incorporated.
Transfer the batter into a springform pan and bake at 200°C for 17 minutes. As a general rule you should take the cake out when the sides look just done but the centre is still a little wobbly. Let sit for 10 minutes before serving.
I have tried making this cake in an "allergy-free" version, making it both gluten and dairy free. I subbed the butter with a plant-based alternative and subbed the flour with a gluten free flour mix. In the gluten-free version I used 85 g of the flour mix.