Line a baking sheet with foil, shiny-side up; smooth out the creases.
Cut the candy bars into pieces. If you're using candies like M&Ms or Reese's Pieces, crush them if you'd like or leave them whole. (They crush easily if you put them in a resealable plastic bag and smack them against the counter, or use a coffee mug to crush them in the bag.)
Create a double boiler by putting a glass or metal bowl over a saucepan of barely simmering water, making sure the bowl doesn't touch the water. Add the chocolate and stir until melted and smooth. Do not overheat the chocolate or it will seize.
Remove the bowl from the pan and wipe the condensation from the bottom. Pour the melted chocolate onto the prepared baking sheet; use the rubber spatula to spread it into a 10-to-12-inch circle, about 1/4-inch thick.
Press the candy pieces into the chocolate, arranging them so each bite has a mix of flavors, colors and textures. Refrigerate the chocolate for 1 hour to completely set before breaking it into large pieces. Break into pieces and store in an airtight container at room temperature for 1 to 2 weeks.