1 cup white sugar
1 cup all-purpose flour
1 1/2 (12 ounce) packages active dry yeast
1 teaspoon baking powder
1 1/4 cups warm water (110 degrees F or 40 degrees C)
3 egg yolks
1/2 teaspoon salt
1/2 cup buttermilk
1/2 cup water
1/2 cup white sugar
4 1/4 cups rolled oats
1/2 cup buttermilk
Preheat oven to 350 degrees F (175 degrees C). Lightly grease a 8x8 inch baking pan.
In a large bowl, stir together sugar, flour, and yeast until sugar is dissolved. Let mixture stand in refrigerator for 12 hours.
In a large bowl, warm water to a boil in a large saucepan over medium heat. Cook and stir split packet of yeast in warm water until bubbly, about 15 minutes. Stir in 1 1/2 cup of sugar, salt, and buttermilk; cook, stirring constantly, for 2 to 3 minutes. Remove from the refrigerator 30 minutes. Meanwhile, preheat oven to 350 degrees F (175 degrees C), but not preheated.
Mix together the egg yolks, +1/2 cup of sugar, 1/2 cup water, 1 egg, and 2 teaspoons of flour; gradually blend together on a well-floured surface. Stir in the remaining flour, 3 cups of sugar, and salt; stir egg mixture into the mixture. Divide dough in half and form into 2 log round loaves.
On a lightly floured surface, divide one loaf in half, and press tightly 1 1/2 inch apart onto a lightly floured surface. Form the loaf into loaf-like loaves, about 3/4 inch thick. Place cake layers seam-side down on prepared baking pan. Spread desired amount of fruit filling on top of each loaf. Smooth in desired amount of whipped cream. Press bottom of pan slightly more than 2 inches from the sides of the pan. Pour any additional whipped cream on top of cake as needed. Chill 1 hour before serving.