Preheat oven to 325 degrees F. Line a heavy duty rimmed baking sheet with parchment paper or a silicone mat. I’ve tested all my batches with a silicone mat.
In a mixing bowl, whisk together the egg white and the oil. Add the muffin mix and stir until you have a very soft dough.
Press the batter into a thin rectangle about 12x8 inches directly onto the lined baking sheet; bake on the center rack for 18 minutes or until dough starts to look set and one or two areas are lightly browned. Remove from the oven and using a pizza cutter, slice into about 12 large triangles or pieces. Separate the pieces slightly, remove any of the pieces that appear browned, and then return the rest to the oven for another minute.
Carefully remove cookie pieces from baking sheet and let cool on a rack. It will crisp as it cools.
Put the chocolate in a microwave-safe bowl and heat at 50% power, stirring every 30 seconds. Drizzle the melted chocolate over the bark and then chill for a few minutes or until set. Repeat with the chopped white chocolate. Drizzle lines of white chocolate over the bark.
Variation: For peppermint bark, use mint flavored chocolate chips.