Just an average gal who loves baking and spoiling her friends and family with her treats.
Actually I did some research about this and I know the answer: it's the powdered sugar. It's been my experience that powdered sugar that's been opened and not properly resealed starts to oxidize very quickly. It can give the sugar a metallic taste that will impart an "off" flavor into your dessert, especially glaze. Acids, such as lemon juice, tend to exacerbate the metallic taste too.
Hope that helps!
First, let me say that I'm in awe of your talent and skill! I am not a professional baker, just someone who absolutely loves to make treats for her friends, family, and the occasional stranger. I really appreciate not just good food, but food that's made sincerely and from the heart. I have to say, and believe me when I say I'm not exaggerating, that *this* cake is one of the very best I've ever made. I cannot believe how flavorful it was! It was just the epitome of what a cake should be and I want to thank you from the bottom of my heart for sharing it with us. If all of the recipes in your cookbook are as good as this one, I'm in big trouble!
I posted my experiences with making your cake on my blog, just in case you're interested. :)
Your newest fan!