In a large bowl, cream butter, granulated sugar and salt until light and fluffy.
Add egg yolks one at a time, beating until smooth after each addition.
Sift flour with cornstarch and baking powder.
Fold into butter mixture.
Knead to a firm dough.
Halve dough; knead rum into 1 half and cocoa powder into the other half.
Place each portion of dough in a bowl and cover with foil or plastic wrap.
Refrigerate 30 minutes.
Preheat oven to 375°F (190°C).
Grease baking sheets.
On a floured surface, carefully roll out light and dark dough separately 1/16 inch (1.5 mm) thick.
Halve each piece.
Place pieces of light and dark dough alternately one on top of the other to make 4 layers.
Press together firmly; cut into small oblong shapes.
Sprinkle each cookie with sugar cystals; press sugar in slightly.
Place cookies on greased baking sheets.
Bake 15 to 20 minutes or until light dough is golden.
Remove from baking sheets with a spatula; cool on a rack