Preheat the oven to 375 degrees. Generously grease and flour an 11-by-17 inch jelly-roll pan.
In a small saucepan over low heat, melt the butter and stir in the sugar and cocoa powder until dissolved. Remove from the heat and let cool, about 10 minutes.
Working with a wooden spoon and a mixing bowl, beat the eggs with the vanilla. Beat in the butter-cocoa mixture. Sift the flour together with the salt and stir into the batter until just blended.
Pour into the prepared pan and spread out evenly. Bake until the top is dull looking and a toothpick barely comes out clean, 20 to 25 minutes.
Remove from the oven, dust with confectioners' sugar, and let cool on a rack. Cut into squares when cool and dust with confectioners' sugar again just before serving.