Simple Shortbread Squares

My favorite Christmas cookie! They absolutely melt in your mouth!

Original recipe makes 120 cookie squares

4 cups all-purpose flour
2 tablespoons all-purpose flour
2 cups butter at room temperature
1 cup white sugar
1 tablespoon colored sugar, or to taste

Preheat oven to 275 degrees F (135 degrees C). Butter a 9×13-inch baking pan.
Mix 4 cups and 2 tablespoons flour, butter, and white sugar together in a bowl using your hands until completely mixed. Spread dough into the prepared baking pan; cover with waxed paper and press firmly into pan. Prick dough through the waxed paper using a fork; remove waxed paper.
Bake in the preheated oven until shortbread is lightly browned, about 1 1/2 hours. Turn off oven and remove pan. Cut shortbread into 1-inch squares. Return pan to oven until oven is completely cooled.
Sprinkle shortbread with colored sugar, remove squares from pan, and store in an airtight container.