In a large saucepan set over medium-high heat, melt the butter. Add the brown sugar and simmer for 4-5 minutes, stirring constantly, until the sugar is melted, caramelized, and fragrant.
6 tablespoons unsalted butter, 1 cup brown sugar
Reduce the heat to medium-low, then whisk in the heavy cream, followed by the milk. Simmer until heated through.
1¼ cups heavy cream, 2 cups whole milk
Place the egg yolks in a mixing bowl. Slowly pour in ½ cup of the hot milk mixture, whisking constantly, to temper the eggs. Add the cornstarch and salt and whisk well, then pour the mixture back into the saucepan and return the pan to low heat.
3 large egg yolks, 3 tablespoons cornstarch, ¼ teaspoon kosher salt
Bring the mixture to a low boil for about 1 minute, whisking constantly, until thickened. (Be careful, the mixture will bubble and pop!)
Remove the pudding from the heat and whisk in the vanilla and bourbon (if using). Allow it to cool for 5 minutes, then divide it into 6-8 dessert cups, press plastic wrap to the tops of each, and chill for at least 2 hours or overnight.
½ tablespoon bourbon, 1½ teaspoons pure vanilla extract
Serve chilled with whipped cream and toffee bits, if desired.
Whipped cream, Toffee bits