Preheat the oven to 350°F/180°C.

In a medium bowl, whisk the sugar and eggs until the mixture is pale yellow in color.

Sift in the flour, cocoa powder, and salt.

Fold until incorporated.

Mix in the butter and vanilla.

Grease a pan with butter and sprinkle cocoa powder to coat.

Pour in batter and smooth out. Batter will be very thick.

Bake for 20 minutes or until the top has hardened. The center should still be soft.

Sprinkle with powdered sugar.