1) Preheat oven to 350 degrees. Butter and flour a 10 inch baking pan.
2) Stir the eggs, sugar and vanilla over low heat until eggs are warm and sugar has melted. Beat for 10 minutes. The mix should end up extremely thick and larger in volume.
3) Sift the flour, cocoa and salt together. Fold in the dry mixture to the eggs in 3 separate portions. Quickly fold in the butter and then pour the batter into the buttered and floured pan.
4) Bake for 35 minutes. Let cool and serve with whipped cream.