Line cookie sheet or jelly roll pan with waxed paper or parchment.
Melt vanilla candy coating in the microwave, per package directions (typically in 30-second intervals, stirring after each interval until melted and smooth). Stir in peppermint extract.
Spread melted vanilla candy coating onto prepared pan until it's about 1/4-inch thick; don't worry about the shape ... you're going to break it into pieces later anyway! Melt semi-sweet chocolate chips (about 45 seconds in the microwave or in a small saucepan over medium-low heat); stir until smooth. Drizzle stripes over the still-warm vanilla candy coating. To marbelize, use a sharp knife to draw lines through the vanilla candy coating perpendicular to the chocolate stripes.
Sprinkle chopped pretzels and candy cane pieces evenly over the top. Gently press into the coating. Chill in the refrigerator for about 10 minutes to set. Break into pieces.