Coat a wood-graining rocker (available at paint-supply stores) with a thick layer of melted white chocolate, and paint stripes onto sheets of acetate (plastic wrap is not stiff enough), rocking the tool and dragging as you go. Refrigerate until set, about 6 minutes.
Pour bittersweet chocolate over white-chocolate woodgrain. Using an offset spatula, quickly but gently spread the dark chocolate to coat the acetate completely, keeping the white chocolate intact. Gently press the acetate, chocolate side down, into the ganache-coated tiers. Refrigerate until woodgrain is hardened, at least 1 hour. Carefully peel off acetate just before displaying cake.
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