By mistake a Milchig cocoa powder mix was used to make hot cocoa in my Fleishig glass cups and one mug. Is there a way to make the glass cups or the mug kosher again, or do I need to throw them out?
The powder mix was put directly into the cup, and then the cup was filled with hot water from the urn. Then some almond milk was poured to top it off.
The custom is not to Kasher glass. See here.
Ceramic coffee mugs cannot be kashered.
Generally, although you consider the cups “Fleishig”, they may not actually be so. It is only Halachicaly considered Fleishig if it came into contact with hot Fleishigs. Unless you clearly know that this happened, you can assume that it did not.
[Even if they were washed in the Fleishig dishwasher, this would not pose a problem, provided that the dishwasher is always run with detergent. See here on our website for more details.]
If this is the case, the glass cups and the mug do not have to be thrown out, but they are now considered MILCHIG for all purposes.