Chocolate Syrup

Bring cocoa powder and 2 cups water to a boil in a medium saucepan over medium heat, stirring constantly. Reduce heat to medium-low. Stir in sugar and salt until dissolved. Simmer, stirring occasionally, 3 minutes. Remove from heat. Add chocolate; stir until it has melted. Pour through a fine sieve into a large bowl; discard solids. Stir in vanilla. Let cool 2 hours, stirring occasionally. Serve warm, room temperature, or cold.