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1
In the top of a double boiler or in a bowl over simmering water, melt the chocolate. When chocolate is nearly melted, remove from heat and stir until smooth. Set aside to cool; return to double boiler only briefly if chocolate begins to set up.
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2
In a mixer fitted with the paddle attachment, beat the butter at medium speed until smooth and pale. Add the sugar; continue beating, scraping down bowl sides often, until mixture is completely smooth, 3 to 5 minutes total. Beat in the egg and vanilla until blended.
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3
Pour in melted chocolate; mix at medium-low speed just until blended, scraping down bowl sides several times.
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4
In another bowl, with a fork, mix together the cake flour, potato flour, cocoa powder, and salt. Add to the chocolate mixture; mix at low speed until just blended. Try not to overmix.
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5
Put the bowl of dough in refrigerator; chill for 10 to 20 minutesuntil firm enough to handle but not too stiff.
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6
Transfer dough to a lightly floured surface. By hand, press dough into a log about 15 inches long and 2 1/2 inches in diameter. (If dough is too tacky, loosely wrap in sheet of plastic wrap to form roll.)
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7
Spread the crystal sugar on a baking tray. Place the log (with plastic wrap removed) in sugar; roll to evely coat all surfaces except ends. Wrap well in plastic wrap; chill until firm, at least 4 hours or up to 3 days. Log can also be frozen.
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8
Position 2 racks in middle of oven; preheat to 350u00b0F. Have ready 2 cookie sheets lined with parchment paper or Silpat nonstick baking mats. Remove dough from refrigerator; let sit for about 10 minutes, until warm enough to slice evenly. Using a thin-bladed chef's knife, cut log into 1/4-inch slices. Transfer slices to prepared cookie sheets, leaving 1-inch space between each.
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9
Bake 10 to 12 minutes, until tops are dull. Cool on wire racks. Can be stored in airtight container up to one week.
