Cut butter into 1-inch pieces and place in the freezer to chill for 15 minutes.
Sift together the flour and sugar into the bowl of a stand mixer fitted with the paddle attachement. Add the partially frozen butter and the salt. Mix on low speed for 2 minutes, or until the butter is reduced to the size of broken walnut meats. Stop the machine and, by hand, pinch flat any large pieces of butter that remain. Turn the mixer on low speed and add the buttermilk all at once. Mix until the dough comes together, about 15 seconds. The dough should be tacky, but not sticky, and still rather shaggy (rough edges). Remove the dough from the bowl and quickly form into a rough disk. Wrap in plastic. Try not to overwork the dough. Chill for at least 1 hour before rolling out.
Remove the dough from the refrigerator and, if necessary, allow it to sit for about 10 minutes or until it is soft enough to roll. On a well-floured surface, roll the bottom crust 1/8 inch thick or less, and 12 inches in diameter. Transfer the crust to a pie dish. Trim off excess and pinch edges of dough around the dish with fingers (or press with a fork) to form the top of the pie crust.
Cover it with plastic wrap and refrigerate it for a minimum of 30 minutes while preparing the filling.
Preheat oven to 325°. In a medium bowl, combine ricotta, salt, sugar, flour, egg yolks and zest. Whisk together for at least 5 minutes, then add in chopped kumquats. Gently fold in the beaten egg whites and pour filling into unbaked pie shell. Bake for about 1 hour. Pie should brown slightly on top and be lightly firm to the touch. Remove from oven to let cool.
For topping/garnish, combine all ingredients and sprinkle on top of cooled pie.