(This recipe requires an instant-read thermometer to check the temperature of the melted chocolate.)
To temper chocolate: Using a microwave, place 3/4 of chocolate in a microwave-safe glass or ceramic bowl (set aside remaining chocolate). Program microwave at 50 percent power. With bowl uncovered, heat chocolate for 30 seconds and then stir with a rubber spatula. Repeat, heating for 30 seconds and then stirring, until almost all the chocolate in the bowl is melted, about 4 minutes total.
Using an instant-read thermometer, check temperature of chocolate. It should be no more than 115°. If it hasn't reached desired temperature, continue to heat (in 10-second increments). Then add reserved chocolate and stir constantly until completely melted and smooth and thermometer reads 88° to 90°. Chocolate is now tempered.
Stir nut mixture into bowl containing chocolate.
Using an offset spatula, spread chocolate in an even layer, about 1/4 inch thick, on the parchment. Sprinkle with reserved 1/2 cup nut mixture. Allow bark to cool until hard, at least 20 minutes. Break into irregular pieces.
Wrap in gift boxes or airtight tins lined with decorative waxed paper, or place in cellophane bags and tie with ribbon. Chocolate bark will last several weeks.