1. In a saucepan, combine sugar, cocoa powder, salt, and cornstarch. Mix well. Add in cold milk, whisk until combined. 

2. Increase heat to medium, stirring constantly until mixture reaches a boil.

3. Allow mixture to boil for 1 minute while stirring constantly. Remove from heat and stir in butter and vanilla.

4. Allow mixture to cool completely, stirring occasionally to avoid a skin forming on the pudding. 

5. Serve chilled or warm.