Combine flour and salt in a large bowl. Add butter pieces and toss in the flour to coat. Using a pastry cutter or your fingers blend butter into flour until butter is in small pebble-sized pieces.

Add buttermilk and using your hands squeeze buttermilk into flour mixture until a dough holds together. Form into a ball, wrap in plastic wrap and refrigerate for at least 30 minutes or up to 1 day in advance.

Heat oven to 375°F and arrange rack in the bottom third of the oven. Roll dough into a 1/4-inch thick circle on a lightly floured surface. Line a 10-inch pie pan or a 9-inch deep dish pie pan with the dough.

Trim dough to an inch overhang and pinch the sides to create a decorative edge. Use any extra dough to fill any cracks.

Set pie plate on a baking sheet and line with a piece of parchment paper or foil. Fill foil with dried beans or pie weights and bake crust on the bottom rack for 15 minutes. Remove from oven, remove foil with beans and return to the oven to bake until bottom is dry, about another 10 minutes. Set on a wire rack to cool.

In a medium bowl whisk together the sugar, flour, cinnamon, salt, ginger, and nutmeg.

Beat eggs in a separate large bowl until smooth. Add sugar mixture and whisk until smooth. add the pumpkin, butter, lemon juice, and buttermilk. Whisk until smooth and well combined.

Pour filling into the pie crust and place crust on the bottom rack of the oven. Bake until the outer edge is set and the middle is still pretty loose (it will set once it is cool) about 35-45 minutes.

Transfer to a wire rack to cool completely then cover loosely and chill until cold.

Combine all ingredients in a stand mixer fitted with the whisk attachment. Whisk until fluffy, white peaks form.

Serve pie chilled with whipped cream.