Line a baking sheet with parchment paper. Set aside.
Add the pecan pieces, sugar, and butter to a nonstick skillet over medium heat. Stir frequently. Once the sugar starts to melt, stir constantly until all of the nuts are coated and all of the sugar has melted.
Spread the nuts onto the prepared baking sheet, quickly separating them with two spatulas. Allow them to cool completely before breaking them apart. Set aside.
In a medium bowl, whisk together the flour, brown sugar, baking powder, baking soda, salt, cinnamon, nutmeg, ginger, and allspice. Set aside.
In another medium bowl, combine the milk, pureed sweet potato, egg yolks, and melted butter. Whisk together until everything is combined.
Add the wet ingredients to the dry, and mix until things are just combined. Set aside.
In a small bowl, whip the egg whites until stiff peaks form. You can do this by hand, but I recommend using an electric hand mixer. Fold the stiff egg whites into the batter.