1position a rack in the center of the oven and heat the oven to 350°. Generously butter a 9 inch square baking pan. Line the bottom of the pan with the square of parchment, butter the parchment, and then flour the bottom and sides of the pan. Tap out any excess flour.
2Cream the butter and sugar until smooth, using a wooden spoon or the paddle attachment of the stand mixer. Add the eggs one at a time, mixing until just incorporated. By hand, whisk in the vanilla and salt. Sift the flour, cocoa, baking soda and baking powder directly onto the batter. Pour in the coffee. Gently whisk the ingredients by hand until the batter is smooth and mostly free of lumps.
3Pour the batter into the pan, spreading evenly with a rubber spatula. Bake until a skewer inserted in the center comes out with only moist crumbs clinging to it, 40 to 43 minutes. Set the pan on wire rack to cool for 20 minutes. Carefully run knife around the edges of the pan, invert cake onto the rack, and remove the pan. Invert again onto another rack and let cool until just warm. Serve at once or wrap in plastic and store at room temperature for up to five days. Makes a 9 inch square cake, or serves 16.