This beer based smoked brisket marinade really kicks up the heat level. It includes three types of capsicum peppers, plus a shot of black peppercorns to boot. A heavy body beer is best for this marinade recipe, but you can use beef broth instead if you don't want the alcohol around.
Go shopping for the ground pepper you'll need first. Ancho pepper powder, chipotle pepper powder and hot paprika are what you'll need. Then of course, the coarse ground black pepper, too.
Combine all of the ingredients together in a large saucepan. Bring to a simmer over low heat, and continue simmering for 5 minutes. This helps release the rich flavor and heat from the different pepper powders. Allow to cool.
Marinate a 4 to 5 pound trimmed brisket flat at least 6 hours. The longer the beef is in the marinade, the more heat and flavor it will absorb. I let 'em soak overnight, flipping the brisket over every now and then so both sides get flavored up nicely. Smoke with oak at 225-240 degrees until slicing tender, about 6 hours. The brisket internal temp should be 180ish when it's done.
For something a little milder you can make the smoked brisket marinade with just half the amount of the four different types of pepper, and marinate for a shorter amount of time, no more than six hours. That way you'll still get the pepper flavors with less of the burning heat.