In a medium-sized saucepan (off the heat), whisk together the sugar, cornstarch, cocoa and salt until no lumps of cornstarch remain. Whisk together the egg yolks and milk in a medium-sized bowl until well combined. Pour half of these wet ingredients over the dry ingredients in the saucepan and stir until completely smooth. Add the remaining half of the milk mixture and stir to combine.
Move the saucepan to the stove and cook over medium heat, stirring constantly with a whisk until the mixture thickens and starts to bubble. Keep scraping the bottom of the saucepan to prevent scorching. Once the first bubble rises to the surface, turn the heat to low and cook exactly 1 minute more, whisking vigorously and constantly.
Remove the saucepan from the heat and whisk in the butter and vanilla. When completely smooth, add the chocolate chips and stir until combined.
Pour the pudding through a sieve or mesh strainer into a medium-sized bowl to remove any lumps. Cover with plastic wrap, pressing the wrap directly onto the surface of the pudding to prevent a skin from forming. Cool in the fridge for at least 4 hours or until completely chilled.
One Last Thing
My secret: Use a sieve or mesh strainer to correct any lumps that may have formed during cooking.