1. Combine the flour, baking soda and salt in the container of a food processor. Start blending while adding the butter cubes. Continue blending while adding the yogurt. Remove the dough and wrap it in wax paper or plastic wrap. Chill thoroughly.

2. Preheat the oven to 400 degrees.

3. Lightly flour a clean surface. Break off pieces of the dough and roll then into 10 to 12 rounds, each about 1 inch thick. Place each round on the floured surface and roll until paper-thin. Sprinkle with sugar or Parmesan cheese. Transfer the rounds to a baking sheet and bake 5 minutes. Turn off the heat and let stand until the wafers are crisp and lightly browned. Remove from the oven and let cool before serving.