• Line a 9×13-inch baking dish with parchment, leaving a couple of inches overhanging on the long sides to use as “handles.” Lightly butter or spray the pan first to help the parchment stay in place.

Make the Base

• Place the semi-sweet chocolate chips in a microwave-safe bowl. Heat on high for 30 seconds, stir well, then continue warming the chocolate in 15-second bursts. As the chips begin to soften, switch to 10-second intervals to avoid overheating the chocolate. (See Note 1)
• Spread the melted chocolate in an even layer in the prepared pan. Arrange the Oreo cookies on top in a single layer— you'll need 4 rows of 6 cookies.

Make the Toffee Layer

• In a large, heavy-bottomed saucepan with high sides, melt the butter, sugar, and salt over medium-low heat. Stir often until the sugar dissolves.
• Once it reaches a steady boil, continue cooking, gently stirring occasionally, until the temperature reaches 300–310°F (hard-crack stage) on a candy thermometer. This will take about 15-20 minutes.
• Remove the pan from the heat and carefully stir in the vanilla; the mixture will bubble up as it hits the hot syrup.
• Slowly pour the hot toffee over the Oreo layer, using an offset spatula to gently even it out. Important: Hot sugar syrup at this temperature can cause serious burns. Pour steadily, avoid splashes, and keep your hands and arms protected. (See Note 4)

Add The Top Layer

• Sprinkle the white chocolate chips over the hot toffee. After a minute or two, they’ll soften enough to spread. Smooth them into an even layer.
• Scatter the chopped Andes mints and crushed candy canes over the top, pressing lightly so the pieces adhere while the chocolate is still warm.

Chill, Cut, and Serve

• Place the pan in the freezer for about 15 to 20 minutes, or until the bark is firm. Lift the slab out using the parchment and transfer it to a cutting board.

  Cut into rustic pieces with a chef's knife. If you prefer cleaner cuts, warm your knife under hot water, dry it completely, and then slice through the chilled bark. Rewarm and wipe the knife as needed.
• Arrange the bark on a platter and serve.