Preheat oven to 300 degrees F (150 degrees C). Line 2 8-inch cake pans with parchment paper.
Place the butter and sugar into a bowl, and mix with a wooden spoon until the mixture is soft, creamy, and thoroughly blended. Stir in the rye and corn flours, and divide the dough into 2 pieces. Press each piece of dough evenly into a prepared cake pan. Using a fork, prick 7 or 8 rows of shallow little holes into the top of each cake. Sprinkle the tops of the cakes with colored sugar, if desired.
Bake in the preheated oven until the tops of the cakes are golden brown, and the edges turn a slightly darker brown, 45 minutes to 1 hour.
Cut each shortbread into 8 wedges while still warm; cool and store in a tightly sealed cookie tin or container lined with parchment or waxed paper.
