Combine the butter, sugar, cocoa powder, and salt in a medium heat proof bowl set over a pan of simmering water. Stir periodically until the mixture is smooth, the butter is melted, and it feels quite hot (but not burning) to the touch. Remove the bowl from the saucepan and set aside until it is just warm. The mixture will appear gritty.
Place a rack in the lower third of your oven and preheat the oven to 325 degrees F. Line the bottom and sides of an 8x8-inch baking pan with parchment paper and set aside.