In a large mixing bowl, cream together unsalted butter and powdered sugar until light and fluffy, about 3-5 minutes. Gradually beat in eggs and milk until incorporated. Mix in flour and salt to form a smooth dough.

Divide the dough into three parts. Color one part with purple gel food coloring, the second with black gel food coloring, and leave the third white.

Roll out the black and white dough into rectangles, stack black on white, then add purple dough with sprinkles. Roll tightly into a log.

Encapsulate the purple log with black and white dough. Roll in black sprinkles, wrap in plastic wrap, and refrigerate for at least 1 hour.

Preheat the oven to 375°F (190°C). Slice the chilled log into ¼-inch rounds, place on a baking sheet, sprinkle with more purple sprinkles, and bake for about 10 minutes.