are you talking about the point? thats the fatty cut of meat on a full brisket. If you mean just the fat cap off of the brisket I believe some butchers will sell that separately. Just ask for beef fat or yes....fat cap.
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If you are looking for fat to make sausage or add to deer burger, the soft fat on the top of a brisket is a poor choice. You want hard fat that will melt when cooking. Such as beef suet or pork back fat. The deckle of a brisket (triangular shaped fat) between the flat and point works well. I trim 20 or so brisket a year for catering and have used a lot of brisket fat for processing.