Line a baking sheet with parchment paper or a silicone baking mat. Measure and set out the ingredients: 8 ounces dark chocolate, 4 ounces green mint baking chips, and 1 cup chopped Andes mint candies.
Melt the 8 ounces dark chocolate: either use a double boiler (set a heatproof bowl over simmering water, making sure the bowl does not touch the water, and stir until smooth) or microwave in 20–30 second intervals on medium-low power, stirring thoroughly between intervals until fully melted and smooth. Stir often and keep the heat low so the chocolate does not overheat.
Melt the 4 ounces green mint baking chips in a microwave-safe bowl: microwave on medium power in 20-second intervals, stirring after each interval until smooth. If the mint chips remain thick, continue in additional short (10–15 second) bursts, stirring until spreadable.
Pour the melted dark chocolate onto the prepared baking sheet and use an offset spatula or the back of a spoon to spread it into an even layer to your desired thickness.
Drop spoonfuls or small lines of the melted mint chips over the dark chocolate across the surface.
Use a toothpick, skewer, or the tip of a knife to drag through the mint chips and dark chocolate to create swirls. Work while the chocolates are still fluid so the swirls form cleanly.
Evenly sprinkle the 1 cup chopped Andes mint candies over the swirled chocolate, pressing lightly so the pieces adhere.
Let the bark sit uncovered at room temperature until fully set, about 1 to 2 hours. Once set, break or cut into pieces and serve.