Cut a piece of 3″ acetate to the length of you cookie sheet. Tack one end of the acetate to the sheet with a bit of melted chocolate. using a spoon or offset spatula, spread a few teaspoons of melted semisweet chocolate along the length of your acetate. Using a woodgraining tool, pull the chocolate across the acetate, rocking the tool back and forth slowly as you go. You can repeat this step if you’re not happy with the effect. Scrape any excess chocolate from the woodgrain tool if necessary. Place the acetate, still on the sheet, in the freezer for 5 minutes, or until set and firm. Remove from freezer. With a spoon or offset spatula, spread the melted milk and white chocolate over the woodgrain pattern in a smooth layer. Try to work quickly and don’t go over areas multiple times, it may smear the pattern underneath. Place the strip back in the freezer for 5 minutes, until just set.