[pete]

I would think a reduction is exactly what you want. What is the purpose of this?

The problem with reducing most beers is that it tends to drive off many of the nuanced flavors and you are left with a lot of bitterness. I have tried this exact thing with Guinness and didn't have much success without having to add a lot of other things to bolster the flavors and mask the bitterness.

 

[pete]

It would really help if the OP let us know what they wanted to use this "extract" for.