Using a permanent marker, draw a 14x5-inch rectangle on each of 2 sheets of parchment paper. Turn parchment over and place on 2 rimmed baking sheets.

Using a fine-point, food-safe brush, brush melted dark chocolate inside each rectangle. Refrigerate until set, about 5 minutes.

Pour half of melted white chocolate on a rectangle. Using an offset spatula, spread white chocolate evenly, using the rectangle as a guide. Repeat procedure with remaining white chocolate. Refrigerate until set, 10 to 15 minutes.

Break into small rectangles, about 5x1½ inches. (They do not have to be perfect.)