In a small saucepan over medium heat, melt 4 tablespoon butter. Remove from heat and add sugar, cocoa and salt. Blend this well. Place back on heat and stir in milk whisking until it just begins to boil. Remove from heat again and add vanilla and 1 tablespoon butter and whip by hand for 3-4 minutes until thick and glossy. Serve warm,Store in airtight container in refrigerator for 1-2 weeks.