Preheat your oven to 350°F (175°C).
In a large mixing bowl, whisk together the sugar, melted butter, and eggs until well combined.
1 1/2 cups granulated sugar, 1/2 cup unsalted butter, 3 large eggs
Stir in the buttermilk, all-purpose flour, vanilla extract, and lemon zest (if using) until the mixture is smooth and uniform.
1 cup buttermilk, 1 tablespoon all-purpose flour, 1 teaspoon vanilla extract, 1 tablespoon lemon zest
Make the streusel topping by combining the flour, sugar, ground cinnamon, chopped walnuts, and salt. Work the cold butter into the mixture, then chill the streusel mixture for at least 30 minutes.
1 cup all-purpose flour, ½ cup chopped walnuts, 1/2 cup granulated sugar, 1/2 cup unsalted butter, 1 teaspoon ground cinnamon, Pinch of salt
Carefully pour the buttermilk filling into the prepared pie crust.
1 prepared 9-inch pie crust
Generously sprinkle the streusel topping over the buttermilk filling, covering the entire surface.
Place the pie on a baking sheet to catch any drips, and then transfer it to the preheated oven.
Bake the pie for approximately 45 to 50 minutes or until the filling is set and the streusel topping is golden brown and crisp.
Remove the pie from the oven and let it cool completely on a wire rack before serving.