Cut up the chocolate bars into smaller pieces, and add to a microwave-safe bowl or glass measuring cup, along with the shortening or vegetable oil. Microwave on high in 30-second intervals, stirring after each interval. Continue until the chocolate is melted and smooth.
Drop the balls one at a time into the chocolate, and using a dipping tool or fork, remove the ball from the chocolate, tapping off the excess chocolate. Place the ball back on the baking sheet and immediately top with chopped nuts. Repeat with the remaining balls.
Once all the balls are done, return them to the refrigerator to allow the chocolate to set.
If you plan to drizzle leftover dipping chocolate over some or all the balls, allow them to chill for 20-30 minutes in the refrigerator. Give the dipping chocolate a good stir and pour it into a small zip lock bag, press the air out, and seal the bag. Snip off just a tiny bit of the corner and drizzle the chocolate in a back-and-forth motion over each ball. Return the pan to the refrigerator again to allow the chocolate to set.
Store in the refrigerator in an airtight container for five to six days. These also freeze well; place in an airtight container and eat within six months.