You will need a graining rocker, a decorating/painting tool used to make faux bois. This will give the chocolate the appearance of grained wood. You will also need a thin sheet of acetate. Both of these items are available at most hardware and paint-supply stores.
- 3 ounces white chocolate, melted
- 6 ounces bittersweet chocolate, finely chopped
Cut a thin piece of acetate into a 13 1/2-by-16-inch rectangle, and place on a clean work surface, with a long side facing you. Coat surface of faux-bois tool well with a thick layer of white chocolate. Starting at the left side and working from top to bottom of acetate sheet, rock coated tool back and forth while dragging it in one swift motion to make veritcal striation. Continue making striations until the entire sheet is covered. Transfer acetate, chocolate side up, to an inverted rimmed baking sheet; refrigerate until set, about 6 minutes.
Meanwhile, temper bittersweet chocolate: Melt two-thirds of the chocolate in a heatproof bowl set over a pan of barely simmering water until it registers 118 degrees on a candy thermometer. Remove bowl from heat, and add remaining chocolate to the bowl; stir with a rubber spatula (avoid using a wooden spoon which can impart other flavors) until the chocolate cools to 84 degrees. Remove any unmelted pieces; discard. Return bowl to pan; stir until the chocolate reaches 88 to 90 degrees.
Immediately transfer acetate to work surface; pour bittersweet chocolate over the top. Quickly spread evenly over the entire surface with an offset spatula. Try not to spread it too much or white chocolate will smear. Use immediately.