Spray a baking sheet with nonstick cooking spray or line with parchment paper. Whisk the oil and extract together in a small bowl.
Slowly stir two-thirds of the bittersweet chocolate in a heatproof bowl set over a saucepan of gently simmering water until almost completely melted. Remove the bowl from the saucepan, wipe the bottom dry, and add the remaining bittersweet chocolate. Stir slowly until just melted. (If it won’t melt completely, return to the simmering water for a minute.) Stir in half of the oil mixture until well-mixed. Set the bowl next to the saucepan. Repeat the melting method with the white chocolate using another heatproof bowl and stir in the remaining oil mixture. Transfer the melted white chocolate to a resealable freezer plastic bag.
Spread the melted bittersweet chocolate on the prepared pan in an even layer. Snip a 1/2-inch hole in one corner of the bag of white chocolate. Pipe lines of white chocolate lengthwise over the bittersweet chocolate, spacing them 1-inch apart. Use a toothpick or the tip of a chopstick to draw lines across the surface about 3-inches apart, dragging the white chocolate into the dark. Sprinkle the surface with the crushed mints.
Let stand at room temperature until set, about 3 hours.
Storage tip: The bark will keep in an airtight container at room temperature for up to 2 weeks.
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