In medium mixing bowl, combine butter and sugar. Whisk until well combined and creamed. Gradually whisk in ½ a large beaten egg and then vanilla extract.

Sift in cake flour and salt. Use silicone spatula to mix until well combined.

In medium mixing bowl, combine butter and sugar. Whisk until well combined and creamed. Gradually whisk in ½ a large beaten egg and then ube extract.

Sift in cake flour and salt. Use silicone spatula to mix until well combined and dough is a solid purple with no white streaks.

Use hands to form each dough into a rectangular block (approximately 5-in. long and 1-¾-in. wide and tall). Tightly wrap in plastic wrap and freeze for at least 30 minutes.

Remove rectangular blocks of dough from freezer. Use sharp knife to carefully cut each block into thirds lengthwise. Swap the middle piece of vanilla dough with ube dough.

With the cut sides horizontal, cut dough blocks into thirds lengthwise one more time. Swap the middle pieces of dough again.

Wrap each checkerboard block of dough with plastic wrap and gently, but firmly, press to make sure the dough pieces stick together. Freeze for at least 15 minutes.

Preheat oven to 350 degrees F. Remove dough from freezer and discard plastic wrap. Slice into ¼-in. to 1/3-in. thick squares.

Arrange cookies on parchment-paper-lined baking sheets and bake for approximately 12-14 minutes.

Remove from oven and transfer to a wire rack to cool.