Add the salt and vanilla to the warm butter mixture, and stir for about a minute to cool down the batter. Next, stir in the egg until well combined.
In a medium-size, microwave-safe bowl, combine the butter, sugar, and cocoa powder. Microwave on high for 30 seconds. Stir the mixture very well, and then microwave for another 30 seconds. Carefully remove the bowl from the microwave.
TO MAKE THE BARS: Preheat the oven to 325°F, and line a 9x5x3-inch loaf pan with parchment paper.