Beat butter, powdered sugar and salt in a large bowl until pale and creamy.
Knead in flour to make a dough.
Divide dough in half.
Knead cocoa powder into 1 half.
Press each half into a ball and wrap separately in foil or plastic wrap.
Refrigerate 30 minutes.
Divide each piece of dough into 5 pieces.
On a floured surface, roll 1 color into 5 long thin rolls.
With the other color, roll 4 pieces into long thin rolls.
Arrange 9 rolls into a checkerboard design, brushing and sealing with a little egg white.
Roll out remaining dough large enough to wrap around assembled dough.
Seal with egg white.
Wrap in foil or plastic wrap.
Refrigerate until firm, about 1 hour.
Preheat oven to 375°F (190°C).
Cut dough into 1/4-inch (5-mm) thick slices; place on a baking sheet.
Bake 10 to 15 minutes or until light dough is golden.
Remove cookies from baking sheet with a spatula; cool on a rack.