If baking the muffins immediately, preheat the oven to 425 degrees. Grease or line 2 muffin tins with paper liners. If allowing the batter to sit, preheat when necessary.
In a medium bowl, sift together the flours, baking powder, baking soda, cinnamon, nutmeg, allspice, cardamom, and salt. Set aside.
In a stand mixer fitted with the paddle attachment on medium speed, combine the butter, sugar and applesauce.
Turn the mixer down to low and add the eggs, one at a time, blending well after each addition. Add the vanilla with the last egg.
With the mixer on low, add the dry ingredients in 3 additions, alternating with the buttermilk in 2 additions.
When the flour is almost completely combined, stop the mixer, raise the head and scrape the batter in the bowl to make sure everything is thoroughly mixed. Remove the bowl from the mixer. Add the grated apple, dates, and muesli, gently folding to thoroughly combine the batter.
Cover and place the bowl into the refrigerator for an hour or overnight.
When ready to bake, preheat the oven to 425 degrees.
In a small bowl, combine the topping ingredients together (like a streusel). Set aside.
Divide the batter evenly in the muffin tins, until almost full. I used a #12 disher that worked perfectly for standard size muffin tins.
Top each muffin batter with about a tablespoon of the topping.
Place the muffin tins in the oven and bake for 5 minutes. Set your timer. (You don’t want to forget them at this high temperature.)
After 5 minutes, without opening the oven door, turn the temperature down to 350 degrees
Allow the muffins to finish cooking until a toothpick inserted in the center of a muffin comes out with a few crumbs. The time will vary according to your oven. Set your timer and begin checking the muffins after 12 minutes, unless you know your oven runs on the hotter side. Mine took 13 minutes after turning down the heat, a total of 18 minutes of cooking time.
Remove the tins from the oven. Wait 5 minutes and remove the muffins from the tins to cool. Enjoy while still warm, or allow to cool completely and store in an airtight container for up to 3 days. (Honestly, these are best the day they’re made.)