17 ounces good-quality white chocolate (such as Lindt or Baker's), finely chopped
30 red-and-white-striped hard peppermint candies, coarsely crushed (about 6 ounces)
7 ounces bittersweet (not unsweetened) or semisweet chocolate, chopped
6 tablespoons whipping cream
3/4 teaspoon peppermint extract
Turn large baking sheet bottom side up. Cover securely with foil. Mark 12 x 9-inch rectangle on foil. Stir white chocolate in metal bowl set over saucepan of barely simmering water (do not allow bottom of bowl to touch water) until chocolate is melted and smooth and candy thermometer registers 110°F. (chocolate will feel warm to touch). Remove from over water. Pour 2/3 cup melted white chocolate onto rectangle on foil. Using icing spatula, spread chocolate to fill rectangle. Sprinkle with 1/4 cup crushed peppermints. Chill until set, about 15 minutes.
Stir bittersweet chocolate, cream and peppermint extract in heavy medium saucepan over medium-low heat until just melted and smooth. Cool to barely lukewarm, about 5 minutes. Pour bittersweet chocolate mixture in long lines over white chocolate rectangle. Using icing spatula, spread bittersweet chocolate in even layer. Refrigerate until very cold and firm, about 25 minutes.
Rewarm remaining white chocolate in bowl set over barely simmering water to 110°F. Working quickly, pour white chocolate over firm bittersweet chocolate layer; spread to cover. Immediately sprinkle with remaining crushed peppermints. Chill just until firm, about 20 minutes.
Lift foil with bark onto work surface; trim edges. Cut bark crosswise into 2-inch-wide strips. Using metal spatula, slide bark off foil and onto work surface. Cut each strip crosswise into 3 sections and each section diagonally into 2 triangles. (Can be made 2 weeks ahead. Chill in airtight container.) Let stand 15 minutes at room temperature before serving.
*Some reviewers cut back on the cream in the dark layer and the peppermint extract