Place the potatoes in a small saucepan; cover with water and bring to a boil. Cook until the potatoes are tender; drain. Mash the potatoes with a fork. Allow to cool.
Preheat oven to 425 degrees F (220 degrees C). Lightly butter a baking sheet.
Combine the cooled potatoes and almonds. Slowly add the sugar while stirring with a wooden spoon. Stir in the lemon zest; continue mixing with your hands. Form the dough into 1-inch balls. Roll the balls in the pine nuts; brush with egg yolk. Form each ball into a crescent shape. Cover each cookie with cornstarch and then brush with egg white. Arrange cookies on the prepared baking sheet.
Bake in preheated oven until lightly golden, 10 to 12 minutes.
