Checkerboard Icebox Cookies
For the butter dough, knead flour, baking powder, vanilla sugar, powdered sugar, salt, egg and butter to a dough using a stand mixer fitted with a dough hook. Knead dough by hand until smooth on lightly floured surface and divide in half. For the chocolate dough, mix powdered sugar, cocoa powder and milk and knead into one half of the dough. Form each dough into a ball, wrap in plastic wrap and refrigerate for about 1 hour.
To form the checkerboard pattern, form each dough half into a rope about 1 cm thick (approximately 1/3 inch thick). Cut 5 strips from the dark dough and 4 strips from the butter dough, all the same length. Brush dough strips with egg white and make three layers of strips. Form the first layer with chocolate-butter-chocolate strips. Form the second layer with butter-chocolate-butter strips. Form the third layer with chocolate-butter-chocolate strips. Stack layers in order. Thinly roll out remaining butter dough, brush with egg white and wrap around checkerboard layers.
Wrap checkerboard dough in plastic wrap and refrigerate for about 4 hours. Line a baking sheet with parchment paper. Cut checkerboard dough into slices 1 cm thick (approximately 1/3 inch thick) and place on prepared baking sheet. Bake cookies for about 15 minutes in oven preheated to 180°C (approximately 350°F).
The two types of dough can be shaped and layered in other patterns as desired, then cut into slices and bake as directed. Let cookies cool completely before serving.