Place hard boiled and raw yolks into the bowl of a stand mixer and mix together with a fork. Add the sugar and mix well on medium. With the mixer on low, add butter and flour a little at a time and mix until smooth. Form dough into a ball, cover with plastic wrap, and rest in the fridge for 2 hours. Heat oven to 350F. Roll the dough out into 3/8 inch thick “snakes” about 4 inches long. Twist each one into an overlapped wreath shape and place on a parchment lined baking tray. Lightly brush with egg whites and sprinkle each with a little sugar. Bake until light golden, about 10 min. Cool on a wire rack.