i wouldn't bother getting a candy thermometer yet, not unless you really feel like you need one. just go by the consistency first, and then by the color.
but before explaining that, first realize that you can take the suggested cooking times only as a vague guide, since we don't know whether the pan they were using was small or large, which would affect the time to a great degree.
here are the visual clues that will lead up to caramelization.
dip a spoon into the syrup, take it out, and watch how it drips.
- after maybe a minute or so of cooking, it'll runoff of the spoon pretty much like plain water, but a few drops on a saucer will feel sticky when cooled.
- then it'll thicken up and run off the spoon more like honey
- eventually, it'll make threads which harden like fiberglass. at this point keep a good watch on it, because it'll soon start to color. for your first time, i'd recommend turning the heat down to low at this point and proceeding slowly.
from this point on, proceed with your recipe paying attention to the color. again, for your first time, i'd recommend not going to too dark an amber, especially if you are using a thick, heavy pan.
some other points to be careful about
- the closer you get to the end, the less advisable it is to stir, as you could end up with one suddenly granulated mass. stir slowly and only when necessary. i wouldn't stir until you add the cream.
- i 'd advise having the cream at room temperature at least and not adding it all at once, maybe a couple of tablespoons, stir slowly & gently, then 1/4 cup, then the rest.
- at a cup of cream to only 1 cup of sugar, i'd expect this to end up being around the consistency of honey; a lot more liquid than usual for caramel custard. you'll want to be very careful pouring in the custard mixture. you may want to put the baking pan in the fridge to firm up the sauce before adding the custard.
finally, this is a sauce to be used as a topping, like for ice cream or cake, etc., not the usual caramel for baked custard. let us know how it comes out. good luck