Start by preparing your pastry cream. Combine milk, salt, vanilla bean paste, and 4 tablespoons of sugar in a saucepan over medium heat. Bring to a simmer and cook until the sugar has completely dissolved ( about 2 minutes) before removing from heat.
In a separate bowl, combine the remaining ¼ cup of sugar with the cornstarch and egg yolks, whisking together until the mixture is smooth.
Slowly drizzle ¼ cup of your warmed milk mixture into the egg mixture while whisking continuously. Continue whisking while slowly drizzling in the warmed mixture until the two are completely combined.
Once your mixture is smooth, return it to your saucepan over medium heat. Stir continuously and cook for about 7 minutes, until the mixture has thickened to the texture of pudding.
Remove from heat and stir in butter. Continue stirring until the butter has melted and the cream is glossy.
Strain the pastry cream through a mesh sieve, cover with plastic wrap, and refrigerate until cool - at least 2 hours.
Once the mixture is cool and ready to serve, whisk until fluffy.