Preheat oven to 325 degrees
In a saucepan over medium low heat, melt the butter.
In a bowl mix together the butter, flour, cocoa, sugar and salt
Gently whisk the two eggs and add them to the mixture along with the vanilla. Stir gently until it is all mixed together taking care not to over mix.
Prepare an 8" or 9" cake tin or springform pan. Line the bottom with a piece of parchment paper and then using a bit of butter, grease the top of the paper and sides of the pan, use a bit of cocoa powder to dust over the butter.
Pour the mixture into the dusted pan
Bake for about 25 minutes but do check it after about 20. You want the top firm and the inside gooey but not jiggly.
Let cool for about 30 minutes and then run a knife around the edges and invert on a cake plate if using a tin. Sprinkle with powdered sugar if desired and garnish with a few berries.