Prepare the strainer:
Set a fine mesh strainer over a medium mixing bowl and set aside for later use.
Mix the dry ingredients:
In a medium saucepan, whisk together the brown sugar, cornstarch, and salt. Slowly drizzle in 1/3 cup of the milk while whisking constantly to ensure a smooth mixture.
Add wet ingredients:
Once the mixture is smooth, whisk in the egg yolks and the remaining milk.
Cook the pudding:
Place the saucepan over medium heat and cook, whisking frequently for 4-5 minutes, until the pudding begins to thicken. When the mixture starts to bubble slightly, reduce the heat to medium-low and switch to stirring with a wooden spoon or spatula. Continue stirring constantly for another 4-5 minutes, scraping the sides and bottom, until the pudding thickens and forms ribbons when drizzled over the surface.
Incorporate the butter and bourbon:
Remove the saucepan from the heat and stir in the butter and bourbon. Mix until the butter is completely melted and well incorporated into the pudding.
Strain and chill:
Pour the pudding through the prepared fine mesh strainer to remove any lumps. Transfer the pudding to a large bowl or individual serving containers. Cover the surface of the pudding with plastic wrap, pressing it directly onto the surface to prevent a skin from forming.
Chill and serve:
Refrigerate the pudding for at least 2 hours until fully set. Before serving, top with whipped cream and a sprinkle of cinnamon if desired.