Preheat the oven to 400°F (200°C). Roast the sweet potatoes until soft, about 45 minutes. Let them cool, peel off the skins, and mash them until smooth.
In a small saucepan, melt ¾ cup unsalted butter (for both layers) over medium heat. Continue cooking, swirling occasionally, until the butter turns golden brown and smells nutty (about 5-7 minutes). Remove from heat and allow to cool slightly.
In a medium bowl, combine the mashed sweet potatoes, ¼ cup of the brown butter, brown sugar, cinnamon, nutmeg, ginger, and dark rum (if using). Mix until smooth. Add the eggs, vanilla extract, and heavy cream. Stir until fully incorporated and set aside.
Roll out the first pie crust and fit it into a 9-inch pie dish. Trim the excess, leaving about ½ inch overhang. Line the crust with parchment paper or foil, fill with pie weights or dried beans, and bake for 10 minutes at 400°F (200°C). Remove the weights and bake for another 5 minutes until lightly golden. Set aside to cool slightly.
Spread the sweet potato filling evenly over the blind-baked pie crust. In a separate bowl, whisk together the remaining brown butter, eggs, sugar, flour, buttermilk, lemon juice, lemon zest, vanilla, salt, and nutmeg. Slowly pour the buttermilk chess pie mixture over the sweet potato layer.
Roll out the second pie crust and gently place it over the pie. Trim any excess crust and crimp the edges. Brush the top with beaten egg and sprinkle with turbinado sugar. Cut a few small slits in the top crust to allow steam to escape.
Bake the pie at 350°F (175°C) for 55-65 minutes, until the top crust is golden brown and the filling is set. If the edges of the crust brown too quickly, cover them with foil.
Let the pie cool for at least 2 hours before slicing to allow the layers to set. Add a dollop of whipped cream on each slice before serving for an indulgent finish.