Trace the bottom of a heart-shaped aluminum pan over parchment paper; cut along the tracing. Spray pan lightly with cooking spray and line with the heart-shaped paper.
Preheat oven to 325 degrees F (165 degrees C).
Combine butter, white sugar, and brown sugar in the bowl of a stand mixer fitted with the paddle attachment; beat until smooth and creamy. Add egg and vanilla; mix until batter is blended, about 50 strokes.
Whisk flour, salt, and baking soda together in a small bowl. Add flour mixture into the batter; stir until thick but spreadable, about 100 strokes. Fold in milk chocolate pieces gently. Pour batter into the prepared pan; spread evenly with a spatula, leaving a 1-inch margin from the sides.
Bake in the middle shelf of the preheated oven until edges are golden brown and middle is set, 18 to 23 minutes. Press any remaining milk chocolate pieces into the still-warm cookie for decoration.
Let cool in pan for 10 to 15 minutes; transfer to a cooling rack and remove the parchment paper.
You can find aluminum heart-shaped pans at most party stores or in the seasonal section of large supermarkets.
You can use waxed paper in place of parchment paper, but tracing is more difficult.
For best flavor use butter instead of margarine or shortening. Do not melt butter in microwave. Let it soften at room temperature.
If batter is too thick, add milk. If too thin, add more flour.
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