Preheat your oven to 350°F. Crush graham crackers with brown sugar in a food processor or zip-top bag. Add melted butter and mix until the crumbs hold together when pressed.
Press the graham cracker mixture into the bottom and sides of your pie dish. Compact it firmly with a glass or measuring cup. Bake for 7-8 minutes to set the crust, then let it cool slightly.
In a large bowl, combine pumpkin puree, buttermilk, eggs, melted butter, sugars, vanilla, salt, and spices. Whisk or mix until the mixture is completely smooth and lump-free.
Pour the pumpkin filling into the pre-baked crust. Smooth the top gently with a spatula. Bake for 45-55 minutes; cover edges with foil after 25-30 minutes to prevent burning. Check doneness with a thermometer—center should read 180-185°F.
Allow the pie to cool on a wire rack for 2-3 hours. The center may wobble slightly but will set as it cools. Refrigerate before slicing and serve with whipped cream if desired.