Melt the semisweet Chocolate chips in a microwave-safe bowl according to package directions. Stir well. Pour the chocolate onto the cookie sheet and spread it to about a 1/4 inch thickness with the back of a spoon.
(One recipe covers 1/2 a cookie sheet. This recipe works well when doubled.)
Melt the white chips in a separate bowl. Stir in the oil, extract and food coloring. Mix well. Pour the mixture over the chocolate layer and spread with the back of a spoon. Draw the tip of a butter knife through the layers to create swirls. Refrigerate until firm, about 30 minutes.
Remove the bark from the pan. Peel off the waxed paper. Set the bark on a cutting board and cut it into 2 dozen bars with a sharp knife. Store in an airtight container in the refrigerator.
