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In a food processor, pulse the flour, cocoa and salt. Add the butter
and process until sandy, about 3 minutes. Add the confectioners'
sugar, milk and vanilla extract and process until a firm dough
forms. Transfer the dough to 2 sheets of plastic wrap and form into
two 7-inch logs, about 1 1/2 inches thick. Wrap in plastic and
refrigerate until very firm, at least 4 hours or overnight.
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Preheat the oven to 350 degrees and position racks in the upper
and lower thirds. Line 2 large baking sheets with parchment paper.
Beat the egg white in a small bowl. Sprinkle the granulated sugar in
a 7-inch square on a sheet of wax paper. Brush the logs with the
egg white and roll in the sugar, pressing to help it adhere. Cut the
logs into 1/4-inch slices and transfer to the baking sheets, about
1/2 inch apart. Bake the cookies for about 20 minutes, until just
firm to the touch; shift the sheets from top to bottom and front to
back halfway through for even baking. Transfer the baking sheets to
racks to cool.
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