In a large bowl, combine the 2 cups of Bran Flakes cereal with 2 cups of boiling water. Allow it to sit for about 10 minutes until softened.
In another bowl, cream together the 1 cup of butter-flavored shortening with the 3 cups of sugar until light and fluffy.
Add the 4 eggs to the creamed mixture, one at a time, mixing well after each addition.
In a separate bowl, whisk together the 5 teaspoons of baking soda, 1 teaspoon of salt, and 5 cups of all-purpose flour.
Gradually add the dry ingredient mixture to the creamed mixture, alternating with the 4 cups of buttermilk.
Gently fold in the softened Bran Flakes mixture and the 4 cups of All-Bran cereal.
Transfer the batter to an airtight container and store it in the refrigerator.
When ready to bake, preheat your oven to 375°F (190°C). Grease your muffin tin or line it with paper liners. Spoon the batter into each muffin cup, filling them about two-thirds full. Bake for 18-20 minutes.
