"This recipe is perfect for those heart-shaped baking pans you always see in the Valentine section of the supermarket or discount department store. The amount of batter is roughly half of the normal bar cookie recipes. Pink, red, and white-colored M&M's make it sweetheart special, but you can always use regular M&M's or your favorite chocolate chips."
You can find these aluminum, heart-shaped pans at most party stores or in the seasonal section of Wal-Mart or Target. Place pan on top of parchment paper and trace out bottom of pan. Cut out heart-shape paper and set aside. Spray heart-shaped pan lightly with spray shortening then fit heart-shaped parchment inside of pan.
Preheat oven to 325F. In medium mixing bowl cream sugars and butter together with spatula. Add egg and vanilla and mix until blended, about 50 strokes. Set aside. In small bowl add flour salt and baking soda and whisk together. Add flour mixture into egg batter and stir until combined, about 100 strokes. Batter should look like thick pancake batter but spreadable. If batter is too thin, add 1 tbsp. of flour. If batter is too thick, add 1 tbsp. of milk.
Gently fold in 1 cup of M&M's. Pour batter onto paper-lined pan using spatula to scrape out excess batter.
Spread batter evenly with spatula, allowing a 1 inch margin from the sides (the baking process will expand the rest of the way).
Place on middle shelf of oven and bake for 18-23 minutes. Cookie is done when edges are golden brown and middle is firm. Remove from oven and press remaining M&M's into warm cookie for decoration.
Let cool in pan for 10-15 minutes before transferring to cooling rack and removing parchment. Can wrap in plastic or return to pan for serving.