Bring the butter to room temperature and cream it with a whisk. I microwave it just until it starts to melt.
Add the granulated sugar and mix until light-colored and fluffy.
Separate the dough into two parts, 80 g each.
Sift the cake flour (for the plain dough) into one of the bowls of dough and fold it in. Combine the cake flour (for the chocolate dough) and the cocoa powder and sift into the other bowl of dough, folding it in.
Here are the two kinds of dough after mixing.
Divide each dough into 3 equal pieces and stick them together, alternating plain and chocolate. Fold the dough in half 4 or 5 times so that the plain and chocolate parts combine.
Divide the marble dough into 2 pieces!
Wrap each piece in plastic wrap and form into cylinders about 2.5 cm width. Refrigerate for at least 1 hour until firm.
Slice to 8 mm and bake in an oven preheated to 180ºC for about 15 minutes.
Story Behind this Recipe
I did some research and came up with this recipe for attractive cookies that won't fall apart when you want to carry them somewhere!
The cookies will look best if you take them out of the oven before the plain parts get too brown! If you use granulated sugar, they'll have more crunch. If you use Japanese white sugar, they'll be crispy but less crunchy. Powdered sugar will give you really elegant cookies. These cookies won't crumble easily, so they're perfect for gifts.