Beat butter until soft and creamy. Add powdered sugar and beat until smooth.
Stir in flour until the mixture is well combined; press the dough together to form a ball. Shortbread dough is typically a little dry and crumbly, but if the dough is too crumbly, add ½-1 teaspoon of milk.
Flatten the dough and place it in a gallon-sized Ziploc bag, close the bag tightly. Use a rolling pin to roll the dough ¼" thick inside the bag. It will form a perfect square inside the bag!
Carefully cut away the sides of the bag. Use a knife or pizza cutter to cut the dough into rectangle shapes.
Place cookies on parchment-lined or ungreased cookie sheets about an inch apart. Place pans of cookies in the refrigerator. The dough must chill for at least 10-15 minutes to prevent the cookies from spreading while baking.
While the cookies are chilling, preheat the oven to 325°F. Bake cookies for 15-18 minutes or till the bottom edges are barely turning light brown.
Let cookies cool on the pan for a few minutes, then remove to cooling racks.
If desired, drizzle cooled cookies with melted chocolate.