Preheat a waffle iron according to manufacturer's instructions.
In a large bowl, whisk together the flour, brown sugar, baking powder, cinnamon, baking soda, salt, and nutmeg.
In a medium bowl, beat together the eggs, sweet potato, milk, butter, yogurt, and vanilla extract.
Add the sweet potato mixture to the dry ingredients and whisk by hand or use a hand mixer to combine just until no lumps remain.
Grease the heated iron with butter. Pour in a ladleful of the prepared batter (2 tablespoons for mini waffle iron and about 1/4 cup for a standard size). Heat until the waffle is cooked through and starting to turn golden. Transfer to a plate and repeat with remaining batter.
Serve immediately topped with butter, chopped pecans, maple syrup, and a sprinkling of ground cinnamon if desired.