Preheat the oven to 175C.
Grease a 25cm x 38cm Swiss Roll tray, and line with baking paper. Take the baking paper quite high on the ends of tray, and allow it to overhang the sides. This makes it easier to lift the tray out of the biscuit. Set the lined tray aside.
Place the butter and sugar into a medium saucepan over a medium heat. Stir gently until the butter melts, then set side to cool slightly.
Sieve the flour and cocoa powder together, then set aside.
Stir the egg, vanilla extract and salt into the melted butter.
Add the flour and cocoa powder into the liquid mixture, and stir until the dry ingredients are incorporated. You will have a smooth batter.
Pour the batter into the lined tray, then use a silicone spatula (or similar) to spread the batter evenly over the base of the tray. It will be quite thin so just try to get it as even as you can.
Bake for approximately 10 minutes, or until the biscuit is dry to the touch and is starting to pull away from the sides.
Remove the tray from the oven, and allow the biscuit wafer to cool in the tin.