Place the granulated sugar and tea leaves in the bowl of the food processor and grind for 1 miute, or untl the leaves are very finely chopped. Add the butter, flour, cocoa and salt and process for about 45 seconds. Scrape down the bowl and break up any large chunks with a spatula. Process for another 15 to 30 seconds, until the dough looks uniformly dark and forms large, shaggy clumps. Dump the dough out onto a work surface and knead gently several times, just to bring it together.
Squeeze the dough into a log about 12 in long and about 1 in in diameter. Gently roll it back and forth until smooth. Don't add flour if the dough is sticky, simply refrigerate the dough for 15-20 minutes to firm up the butter, then try again.
If you like, sprinkle the sanding sugar on the work surface alongside the log and gently roll the log into the sugar, turning to coat evenly. (I simply sprinkled sugar on after cutting) Cut a piece of plastic wrap several inches longer than the log and center the log at one long edge of the wrap. Roll the log into the wrap so it is tightly bound by the plastic. Twist the ends of the wrap to secure the log and help to create a rounded shape. Refrigerate for two hours.
Preheat oven to 300 degrees and position an oven rack in the center. Line the baking sheet with parchment paper.
Remove the plastic wrap from the dough log and slice it into 3/8 inch thick rounds. Place cookies 1 inch apart on the prepared baking sheet. Bake, rotating the sheet halfway through the baking time, for 30 minutes or until cookies are cooked through and look dry on top. Transfer to a cooling rack and cool completely.
Store in an airtight containter at room temperature for up to 1 week.