Adapted from Cooks Illustrated. Makes about 1/3 cup.
1 Tbsp unsweetened cocoa
4 tsp ground cumin
2 tsp ground allspice
4 tsp black peppercorns
2 tsp kosher salt
Grind all ingredients in a dedicated spice grinder (don't use your coffee mill) or mortar until no whole peppercorns remain.