Heat oven to 400°F. Combine flour, sugar and salt; blend thoroughly. Add oil and water to dry ingredients and mix thoroughly. If using butter, cut into flour mixture until it resembles coarse cornmeal. Add water and blend thoroughly.
Shape into balls using approximately one tablespoon of dough. Flatten dough to form circles. Place a generous teaspoonful of koshi or tsubushi an in center of each circle and pinch edges together to seal.
Place on ungreased cookie sheet. Bake for 30-35 minutes or until golden brown. Serves 12.
Variations: Substitute the koshi or tsubushi an with apple pie filling.
Approximate Nutrient Analysis per serving (based on oil):
320 calories, 19 g fat, 2 g saturated fat, 0 mg cholesterol, 100 mg sodium, 36 g carbohydrate, 1 g fiber, 9 g sugar, 4 g protein