In a saucepan, whisk together the sugar, cornstarch and salt. Add in the egg yolks and whisk until the mixture comes together and is combined. Turn the heat to medium-low and stream in the milk while whisking continuously. Bring the mixture to a bowl, whisking the entire time, then reduce it to a simmer for one minute and continue to whisk. It should thicken. Place a fine mesh strainer over a bowl and pour the filling through the strainer. Whisk in the chocolate, butter and vanilla bean paste until it all comes together and is a thick and delicious chocolate pudding. Cover the surface with a piece of plastic wrap and let cool completely for a few hours.