Mise en place. Prep rimmed cookie sheet (jelly roll pan) with parchment paper. Use shortening to help it stick.
Melt 3/4 of semisweet chocolate for 30 seconds at 50% power in microwave. Stir. Heat again in 20 second cycles (50% power) until chocolate melted. Add rest of semisweet chocolate. Heat another cycle if needed.
Mix in 1/2 tsp. peppermint extract. Spoon evenly (you want a thin layer!) into prepared pan.
Break up the Oreos. Spread 6-8 cookies over melted chocolate. Press down lightly. Sprinkle 1/4 cup of peppermint crumbles over melted chocolate.
Refrigerate for 20 minutes.
Melt 3/4 of white chocolate for 30 seconds at 50% power in microwave. Stir. Heat again in 20 second cycles (50% power) until chocolate melted. Add rest of white chocolate. Heat another cycle if needed.
Mix in 1/2 tsp. peppermint extract. Spoon evenly over chilled semisweet layer. Tilt pan back and forth to completely cover bottom layer.
Sprinkle rest of Oreos over top of white chocolate. Press down lightly. Sprinkle 1/4-1/2 cup of peppermint crumbles over melted chocolate.
Chill in refrigerator 60 minutes (or until chocolate hardened). Break into pieces.
Notes
Be very careful melting chocolate. The bowl should never be too hot to hold in your bare hands. If you overheat, the chocolate clumps and becomes scorched. If in doubt, allow it to cool slightly before another heating cycle.