Preheat the oven to 250 degrees F. Spray two 18-by-13-inch rimmed baking sheets with nonstick spray.
Prepare the cake batter according to the package directions and divide it between the prepared baking sheets. Bake until the tops are dry and beginning to crack, about 1 hour and 15 minutes. Let cool completely.
Drizzle with the melted chocolate and top with the sprinkles. Let the chocolate harden at room temperature, about 45 minutes.
Break the cake into chip-size pieces. Serve with the frosting on the side for dipping.
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