Pierce the sweet potatoes all over with a fork and bake in the oven at 400 degrees F for 45 minutes or until soft. Let it cool a few minutes then scrape out the flesh.
Run the sweet potato flesh through a potato ricer. Place it in a small-medium pot with the sugar. Stir over medium heat until the sugar is dissolved to liquid. Add the lemon zest, orange zest and candied ginger. Increase the heat to medium-high and simmer the mixture for 5 minutes, stirring constantly to prevent scorching. Remove from heat and add all remaining ingredients except for the eggs, orange blossom water and egg yolk. Stir to thoroughly combine. Then add the eggs and orange blossom water and stir to thoroughly combined. Let the mixture cool, cover with plastic wrap and refrigerate overnight to let the flavors meld and the dough firm up.
Preheat the oven to 375 degrees F. Use a spoon to scoop out the dough and roll into balls about 1 1/2 to 1 3/4 inches in diameter. Then shape each ball into an oval with tapered ends. Place the cookies on a lined baking sheet and gently press them just to very slightly flatten them (see images in blog post) and brush them with the egg yolk. Bake for 15-20 minutes until lightly browned. Let cool completely.
Makes about 30 cookies. These freeze well in an airtight container or ziplock bag.
*Yes, that really is 3 1/2 cups of sugar, which is actually less than what many traditional recipes call for!