Using a hand mixer or stand mixer fitted with a paddle attachment, cream together butter, sugar, vanilla extract, and salt until well incorporated.

Divide the dough into two balls evenly. Add 1 cup flour to a bowl and mix until well combined with a spatula to make the vanilla shortbread cookie dough.

Add 3/4 cup flour and cocoa powder to another bowl and mix until well incorporated with a spatula to make the chocolate shortbread cookie dough.

If the dough is too sticky to handle, refrigerate it for 20 minutes until just a little bit firmer.

Take half of the vanilla cookie dough and flatten it with your hand into a rough 6x3-inch/15x7.5 cm rectangle. Put the flattened dough onto a floured work surface. Take half of the chocolate cookie dough and flatten it again into the same size rectangle. Put the flattened chocolate cookie dough on top of the flattened vanilla cookie dough. Repeat the entire process so you have 4 alternating layers. (See the video for the visual guide)

Cut the layered dough vertically in the middle with a sharp knife. Then, put one part overlay the other. Press and pat the dough into a rough 6x3-inch/15x7.5 cm rectangle. Cut in the middle and stack the dough again. Press the dough into a log about 2-inch/5cm wide. (See the video for the visual guide)

Wrap the dough with parchment paper or plastic wrap. If the dough is too long, you can divide it into two and wrap them individually. Chill until very firm for at least 2 hours.

Slice the log into a 1/8 to 1/4-inch or about 3-5mm thick with a sharp knife. Arrange the slices on the baking pan, spacing 2-inch apart.

Bake for 13-15 minutes or until the bottom edges of the cookies are golden brown in color. Rotate the baking sheets halfway through for even baking.

Transfer to a wire sheet to cool completely.