Step 1

Whisk the cornstarch with the salt in a small bowl. Slowly add the milk and whisk until the mixture is smooth. Make sure there are no lumps.

Step 2

Whisk in the egg yolk and set the bowl aside.

Step 3

Melt the butter in a deep saucepan set over medium-high heat. When the butter has melted, whisk in the brown sugar.

Step 4

The brown sugar will look grainy at first, but whisk vigorously until the brown sugar completely melts and becomes a liquid.

Step 5

Remove the pan from the heat.

Step 6

Slowly whisk in the cream. (The sugar and cream will steam but continue to whisk. The sugar will clump together, this is normal). Return the pan to the stove, reduce the heat to low and continue to whisk until the sugar has melted and is smooth.

Step 7

Very slowly add most of the hot butterscotch liquid to the bowl with the cornstarch and milk and whisk. (You want to add the hot butterscotch slowly because you don't want the heat from the butterscotch to "cook" the eggs.)

Step 8

Return the tempered cornstarch slurry to the pan and cook. Raise the heat to medium-high and whisk vigorously until it comes to a boil.

Step 9

Continue whisking and cooking for an additional 2 minutes. Remove the pan from the heat and whisk in the vanilla.

Step 10

Carefully spoon into a bowl or a glass and chill for at least an hour.

Step 11

Top with whipped cream, if desired.