How can I thicken/firm up this caramel? At the weekend, I made millionaire shortbread according to the recipe in Nigella Lawson's &quot;How To Be a Domestic Goddess&quot; (the full recipe is below for completeness). For the caramel part, Nigella uses a microwave and the instructions are as follows:

200g butter
397g sweetened condensed milk
4 tbps (100g) golden syrup
Melt 200g of butter in the microwave (in a large microwavable bowl)
for 2-3 minutes, then add the condensed milk and golden syrup. Whisk
the mixture well until the butter is thoroughly incorporated. Heat for
6-7 minutes until it is boiling, stirring thoroughly every minute.
[...] It's ready when it's thickened and turned a light golden-brown.

Now the result of this is delicious, but it is far too soft for millionaire's shortbread, and it squishes out the sides when trying to eat or cut. My partner has also made this recipe with the same results.
I don't really understand the science of caramel, so I'm not sure what to change to get the results I want. So I have a few questions:

What can I change? Is there something in the above recipe I can do more of, do less of, add more of, etc.?
The answer to this question suggests that the above ingredients are more reliable (than standard caramel) for getting the right texture, but the linked recipe does not use the microwave. Might I get better results without the microwave?
In Nigella's foreword, she advocates the use of the microwave as &quot;what would normally take a good couple of hours takes only a few minutes&quot;. Caramel does not take a couple of hours, does it? What is she talking about?
The problem with quick caramel recipes is that they aren't reliable due to the variations of water content in the butter and brands of condensed milk. Butter can vary between 15-30% water, if your butter has more water the caramel will be looser.
The way to fix it it to simply cook it down longer. I personally think the microwave method is a bit gimmicky, all the opening and closing of the door and stirring doesn't save much time or effort. I use the same ingredients and cook them on the stove as it is the condensed milk which is the secret to the quick caramel, not the microwave. Cook it high to get the color, then low to thicken it until you get the right consistency. A wide pan helps with this as it increases the surface area for evaporation. It really doesn't take that much longer, just a few minutes in my experience.
If you are in a hurry reach for the cornstarch and add in small amounts of milk cornmeal slurry until you get the consistency you want. This will do the job but won't have the same strength of flavor as if you cook it down to thicken it.