Line a baking sheet with parchment paper.Melt semi-sweet chocolate until smooth. Stir in half of the peppermint extract and optional oil. Spread onto the sheet.Let the layer partially set until tacky.Melt white chocolate and stir in remaining peppermint extract.Pour white chocolate over the dark layer and spread gently.Sprinkle crushed peppermint candies on top and press lightly.Let the bark cool completely, then break into pieces and enjoy.