* ブラウニー (burāunī): This is the most common and straightforward way. It's a loanword directly from English, pronounced phonetically. This is generally understood.
* チョコレートブラウニー (chokorēto burāunī): This translates to "chocolate brownie," which is more descriptive and avoids any potential ambiguity.
Therefore, ブラウニー (burāunī) is the simplest and most commonly used term, but チョコレートブラウニー (chokorēto burāunī) is a safer bet if you want to ensure everyone understands you're talking about the chocolate dessert.