Heat the oven to 400 degrees. Prick the sweet potatoes with a fork and place on a foil-lined cookie sheet. Roast for 1 hour or until they are tender when pierced with a fork and are caramelizing. Remove from the oven and allow to cool, then lightly mash with a fork. Set aside.
Lower the oven temperature to 350 degrees. Lightly spray the muffin tin with vegetable oil.
Cream the butter and sugars until light and fluffy, about 3 minutes.
In a medium bowl, sift together the flour, whole-wheat flour, whole-grain pastry flour, baking powder, baking soda, salt, cinnamon, ginger and allspice. In a separate bowl, whisk the buttermilk, yogurt, egg and vanilla together.
Scrape down the sides of the butter bowl and alternately add the dry and wet ingredients; do not overmix. Gently fold in the sweet potatoes, then the dates.
Using an ice cream scoop (about one-half cup capacity), scoop the batter into each of 10 prepared muffin cups, about 1 scoop per muffin. Bake for 35 to 40 minutes. The muffins will be dark golden brown on the bottom.