Line a 13 x 9-inch baking dish or tray with parchment paper.
Melt the semi-sweet chocolate according to the package instructions until smooth. Pour it onto the prepared tray and spread it into an even layer.
Allow the chocolate to harden completely before continuing. For faster results, place the tray in the refrigerator.
Melt the white chocolate according to the package instructions. Pour it over the hardened semi-sweet chocolate layer and spread evenly. Let the white chocolate set until firm.
Melt the red, white, and blue candy melts separately according to the package instructions. Drizzle the melted candy over the white chocolate layer using a spoon, piping bag, or zip-top bag with a corner snipped off.
Use a toothpick or skewer to gently swirl the colors together, creating a marbled design. Work before the candy coating begins to set.
Immediately sprinkle the top with red, white, and blue star sprinkles and M&Ms so they adhere to the melted candy.
Let the bark harden completely at room temperature or refrigerate until set.
Lift the bark from the tray using the parchment paper and break it into pieces by hand before serving.
Notes
Let each chocolate layer set completely before adding the next to keep the layers neat and distinct.
Swirl the candy melt colors as soon as they are drizzled on top for the best marbled effect.
Change up the candies and sprinkles to match different holidays, parties, or color themes.
Refrigerate the bark between steps or after decorating to help it set more quickly.