If using an ice cream maker, make sure that you put the ice cream bowl in the freezer so that the interior is completely frozen by the time you are ready to churn the ice cream.

Melt butter in a medium saucepan over medium-low heat. Add the brown sugar and whisk to combine. Cook, stirring occasionally, until the sugar has dissolved and begins to bubble, about 4 minutes.

Add ½ cup of the cream to the pan (the mixture will bubble) and whisk until the mixture is smooth. Continue to cook, whisking frequently, for 5 minutes, until the mixture has thickened and is very smooth. Whisk in the milk and turn the heat to low.

In a medium bowl, whisk egg yolks, salt and vanilla. Whisking constantly, slowly add ⅔ of the hot butterscotch mixture to the yolks, whisking until smooth.

Add the yolk mixture back to the saucepan and cook over medium-low heat, whisking constantly, until the mixture has a custard consistency and is thick enough to coat the back of a wooden spoon, 3-4 minutes. Whisk in the remaining ½ cup cream and the bourbon.

Transfer the custard to a heat-proof bowl and cover with plastic wrap. Chill until the custard is very cold, at least 3 hours or overnight.

Churn the ice cream according to the instruction for your ice cream maker, until it has thickened and has the consistency of soft-serve (20-25 minutes). Transfer the ice cream to a freezer container, cover, and freeze until firm about 4 hours or overnight.

Pour the custard into a freezer container (I used a loaf pan) and cover tightly with plastic wrap. Freeze for 1 hour.

Remove the pan from the freezer and uncover. Using an immersion blender or a hand mixer, thoroughly mix the custard for about 1 minutes, making sure that you get into the corners of the pan. This helps to aerate the ice cream, mimicking the process of an ice cream maker.

Repeat the process 2 more times after hour two and hour 3 in the freezer, mixing thoroughly each time.

Return the pan to the freezer, cover, and freeze until firm, another 4 hours. Give the ice cream a final mix, and then serve or cover and freeze overnight to firm up even more.