I'm trying to make white chocolate blondies by changing a chocolate brownie recipe by replacing the dark chocolate with white chocolate. I made the regular chocolate version yesterday, but today when I melted the butter & white chocolate together they separated! I thought by adding in my sugar and stirring the mixture might come together, but it hasn't. The clear part of the butter just pools on top! What can I do to save it???

FYI: I've put it in my mixer with the whisk attachment and beat it for about 5 mins, but it still hasn't come together. I hope I haven't wasted 11oz of god quality white chocolate and 1 cup of butter

That can happen if you do it on too high of a heat. I generally melt my butter in the pan on med-low heat, then add the chocolate chunks off the heat and stir til it's all melted. You can put it back on the heat on low if there are any stubborn chunks.
I don't know if there's anything you can do to save it. Since you've made the mix anyway, I would bake it and see what happens before throwing it all away...
Also, you might find that just subbing one chocolate for another won't work, if you leave all the other ingredients the same. Blondies need less sugar than brownies, since white choc is so much sweeter than dark. Try it and see though!

That can happen if you do it on too high of a heat. I generally melt my butter in the pan on med-low heat, then add the chocolate chunks off the heat and stir til it's all melted. You can put it back on the heat on low if there are any stubborn chunks.
I don't know if there's anything you can do to save it. Since you've made the mix anyway, I would bake it and see what happens before throwing it all away...
Also, you might find that just subbing one chocolate for another won't work, if you leave all the other ingredients the same. Blondies need less sugar than brownies, since white choc is so much sweeter than dark. Try it and see though!

Could the butter have had too high of a water content? Cheaper butters usually have more water in them.

From all the recipe I have seen Blondies are more like a thick cookie bar and not really the same texture as brownies. They also usually have chocolate chips add, not melted chocolate mixed in the whole batter. If you want to make a white chocolate brownie you may want to look up recipes for that.

As far as what you are trying to do, I'd probably go ahead and finish mixing it. The extra liquid will probably blend in fine with the rest of the stuff.

To me, a blondie is similar in texture to a brownie, but it's flavored with brown sugar instead of chocolate. As TexasSugar said, they often have chocolate chips (and/or white chocolate chips) mixed in.

Your white chocolate brownies might bake up okay, even if the batter separated. My regular brownie recipe often separates when I combine the oil melted butter with the chocolate/cocoa/water mixture, but it evens out when I add the eggs. They always turn out great when they're baked.