Butter a small square pan and set aside. In a heavy pan mix cocoa, sugar, milk and salt. Mix well and put on high heat till boiling, stirring constantly. Let simmer till a soft ball forms (after boiling starts - about 15 min.) in a cold glass of water.
Then add vanilla and peanut butter. Stir until it starts to thicken a little.
Pour into a buttered pan. Let cool and cut into small squares.