Microwave the semi-sweet chips in a large glass bowl for 1 minute on High. Stir. Heat again in 30 second intervals, stirring between, until fully melted and smooth. Spread evenly in the jellyroll pan. Tap pan to release air bubbles and even out the chocolate. Cool in refrigerator.
Place the mints in a heavy Ziploc-type bag and crush them using a rolling pin until the pieces are small and uniform. Sift the peppermint “dust” from the crushed pieces and reserve.
Microwave the white chips in a glass bowl for 1 minute on High. Stir. Heat again in 30 second intervals, stirring between, until fully melted and smooth. Add the peppermint oil and blend completely. Stir in peppermint “dust” to taste and/or based on how pink you want your bark.
Once the dark layer is cool and firm (not rock hard), score it with a pattern using a knife or pierce it all over with a fork in a regular pattern. This will help the white chocolate mix adhere to the dark layer. Spread evenly with the white chocolate mix, and then tap the pan to release the air and even out the chocolate. Sprinkle the crushed peppermint pieces over the white layer, gently pressing the pieces to imbed them into the chocolate. Cool in refrigerator.
When the white layer is cool and firm, carefully invert the bark onto another sheet of parchment paper and return it to the refrigerator. When the dark layer is cool and firm (not rock hard), remove the parchment paper sticking to it and let the bark sit at room temperature for 10 minutes.
Using a pizza cutter or sharp knife, score the dark layer crosswise into 12 equal strips. Go over each score mark individually, pressing harder each time until the strip breaks off. Cut each strip into 1 inch pieces.
Lay the pieces in a single layer and return to a cool place until ready to use. Package up the “pretty” pieces for gifts, and keep the “rejects” to eat!