10 ounces white chocolate, coarsely chopped (2 cups)
Melt milk chocolate in a heat-proof bowl set over a pot of simmering water, or in a microwave. Just before all is melted, remove from heat; stir with a spatula until completely melted. Repeat with white chocolate.
Cut a 12-by-9-inch sheet of parchment; place on a baking sheet. Transfer 2 tablespoons milk chocolate to a piping bag fitted with a small round tip (such as Ateco #3; you can also use a tiny paintbrush). Pipe or brush small dots and dashes and "knots" sparsely across surface of parchment. Refrigerate 5 minutes.
Pour white chocolate over pat- tern, spreading to edges of parchment. Refrigerate until set, about 15 minutes. Spread milk chocolate over white chocolate with a spatula. Refrigerate until set, about 15 minutes. Break into pieces; refrigerate until ready to serve.