First, make the pudding: In a microwave-safe measuring cup or bowl, heat 1 1/4 cups heavy cream on High for 30 to 45 seconds or until warm but not boiling. Set aside.
In a large, heavy saucepan over medium-low heat, melt butter. Add brown sugar; stir constantly until sugar is dissolved and mixture begins to bubble, about 3 to 5 minutes. Gradually stir in warmed cream. Remove from heat.
In a small bowl, whisk cornstarch with salt. Whisk in 1/2 cup milk until mixture is smooth. Pour cornstarch-milk mixture into saucepan along with remaining 1 1/2 cups milk. Cook over medium heat, stirring constantly, until mixture is thickened, about 4 to 6 minutes. Off heat, whisk in egg yolks, 1 1/2 teaspoons vanilla extract and bourbon, if using.
Return pudding to heat and bring to a boil, stirring constantly. Remove from heat; whisk in vinegar. Strain pudding through a fine-mesh sieve into a medium bowl. Pour pudding into 6 parfait glasses, small drinking glasses or dessert bowls. Press a piece of plastic wrap directly on surface of pudding in each glass.
Refrigerate pudding until well-chilled and thickened, about 4 hours up to overnight.
Before serving, make the topping: In the bowl of a stand mixer with whisk attachment or in a large bowl using an electric hand mixer, beat 1/2 cup heavy cream with 1/4 teaspoon vanilla extract and 1 tablespoon granulated sugar on medium-high speed until stiff peaks form. Spoon whipped cream on top of chilled pudding in glasses. Sprinkle chopped toffee on top as desired. Serve immediately.
Store pudding covered in refrigerator for up to 3 days.