In a medium bowl, cream together butter or margarine and white sugar until light and fluffy. Blend in egg and vanilla. Sift together flour and salt. Add to creamed mixture and mix well. Stir in oats.
Divide dough in half. Add peppermint extract and a few drops of food color to half of dough. Chill for several hours.
Preheat oven to 350 degrees F (175 degrees C).
Roll out each half of dough to 1/8 inch thickness on surface sprinkled lightly with confectioners' sugar. Cut out half of each color with the 2 1/2 inch heart shaped cookie cutter, and half with a 1 1/2 inch cookie cutter. Try to make it so that you have an equal number of small and large hearts in opposing colors. Place them on unprepared cookie sheets.
Bake for 10 to 12 minutes in the preheated oven.
Remove from oven and place 1 one square (1/8) of a chocolate bar onto the center of each large heart. Place small heart on top and press lightly to secure. Transfer to rack to cool.