Melt the chocolate. Place the peppermint sticks in a heavy, locked plastic bag and crush them fine by whacking and then rolling them with a rolling pin. They should have about the same consistency as crushed ice. When the chocolate is melted, stir in the peppermint oil. Stir in all but 2 tablespoons of the crushed peppermint pieces and spread the mixture out on the cookie sheet, about 1/4-inch thick. It will not fill the pan completely. Sprinkle the reserved peppermint stick over the top. Let set at room temperature until hardened, about 2 hours. (Or, refrigerate for 30 minutes to harden more quickly.) Use your hands to break into pieces. Store in an airtight container for up to 2 weeks.