Preheat oven to 350º. Melt butter in a small saucepan over low heat. Remove from heat and add brown sugar. Stir until smooth. In a medium bowl, combine all-purpose flour, whole-wheat flour, baking soda, oats, and salt. Combine the butter mixture with the dry ingredients, and add egg, vanilla, and chopped dates. Fold in bittersweet chocolate. Mix well and spoon mixture by tablespoon-fulls out onto lightly greased (or silicone baking mat-covered) baking sheets. Bake for 12 minutes, until tops are dry to the touch.