Beat butter until soft and creamy. Add powdered sugar and beat until smooth.
Stir in flour until the mixture is well combined; press dough together until it forms a ball.
Flatten the dough and place it in a gallon-sized Ziploc bag. Close the bag tightly. Using a rolling pin, 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 rectangular shapes.
Place cookies on ungreased cookie sheets about an inch apart. Place pans in the refrigerator.
While the cookies are chilling, preheat the oven to 350°F. Bake cookies for 15-18 minutes or until the bottom edges are barely turning light brown.
Let cookies cool on the pan for a few minutes, then remove to cooling racks.
Melt chocolate according to package directions.
Dip half the cookie into the chocolate, shake off the excess, and place them on parchment paper to set up. Add sprinkles or coarse sugar right after dipping, if desired.
Let dry all the way before eating or gifting.