Tell me how to thicken this chocolate sauce!

Tell me how to thicken this chocolate sauce!

I made the Epicurious chocolate sauce that was recommended to me by Sooz (thanks!) about a month ago and I liked the taste and the simplicity. All the ingredients were things I always have on hand anyway. It was SO fast to make, it took less time than me reading the labels on the hot fudge sauces at the grocery store.

Epicurious and America's Test Kitchen (it's the very same recipe):

Chocolate Fudge Sauce

1/3 C cocoa, measured, then sifted, set aside

3 oz chopped unsweetened chocolate, set aside (don't make chunks out of your chocolate because they take too long to melt)

4 T unsalted butter, cut into little cubes, set aside (put it in the fridge until needed)

1 tsp vanilla, set aside

In sauce pan:

2/3 C whole milk

1 1/4 C granulated sugar

1/4 tsp salt

Put the sauce pan over medium heat and whisk while heating--this will take 3 to 5 minutes, and then it will come to an active boil.

Add 1/3 C of cocoa and whisk it in with the milk, sugar and salt.

Remove the sauce pan from the heat.

Add 3 oz chopped unsweetened chocolate that you set aside, and whisk it in the milk, sugar, salt and cocoa in the saucepan... it'll melt right in!

Let it sit for 3 minutes.

Whisk.

Add 4 T unsalted butter & whisk in the butter.

Add 1 tsp vanilla and whisk.

But it wasn't thick enough for my tastes. It was thick out of the refrigerator, but I knew that warming it up to pour over ice cream would make it thinner than I like.

Would it be thicker if I substituted whipping cream for the milk? Or just reduced the volume of the liquid? Thanks.