Place a disposable slow cooker liner in a 3-1/2- or 4-quart slow cooker. Add candy coating, white chocolate, and shortening; stir to combine.
Cover and cook on low-heat setting for 1 to 1-1/2 hours or until mixture is melted and smooth, stirring every 30 minutes. Stir in peppermint extract. Stir in pretzels and peppermint candies.
Line two large baking sheets with parchment paper. Drop candy mixture by heaping tablespoons onto the prepared baking sheets. Let stand at room temperature until firm.
In a small saucepan cook and stir dark chocolate over low heat until melted; cool slightly. Spoon melted dark chocolate into a decorating bag fitted with a 1/4-inch plain tip. (Or fill a heavy resealable plastic bag with melted chocolate; snip a small hole in one corner of the bag.) Drizzle melted dark chocolate over candies. If desired, sprinkle with additional crushed peppermint candies. Let stand at room temperature until firm.