In a small bowl, sift flour and cocoa together. Add coarse salt and set aside.

With an electric mixer beat butter on medium speed for 3 to 5 minutes until fluffy. Add sugar and continue to beat until very light in color, about 2 minutes more. Add the flour and cocoa mixture and combine on low speed, until flour is just incorporated and dough sticks together when squeezed with fingers.

On a sheet of wax paper, pat and form the dough into a log about 12 to 14 inches long and 1 to 1 1/2 inches in diameter. If you want a square or rectangular cookie rather than round, flatten each side of the log as you roll it up in the wax paper. Chill until firm, at least 1 hour.

Preheat oven to 325 degrees.

Line a cookie sheet with parchment. Remove the wax paper from the dough and slice the log into 1/4 inch thick slices. Lay cookies on lined baking sheets. Bake until firm, 17 to 20 minutes. Cool completely on wire racks. Can be stored in an airtight container for 3 to 4 weeks.