Brownie Question - What Is This Taste?

It is probably the bitterness from the unsweetened chocolate. If you did not premelt the Ghiradelli chocolate, there could have been a chunk left in the brownie you ate, try another bite. That is all I can think of. Try some of the unsweetened cocoa if you have never done so, you will see that it is bitter. Do that to compare the tastes and see if it is it.