Preheat the oven to 375°F (190°C). Roll out the bottom pie crust and carefully press it into a 9-inch pie plate, ensuring it fits snugly against the edges. Trim any excess dough and place the pie plate in the refrigerator while preparing the filling.

In a large saucepan over medium heat, combine the elderberries and sugar, stirring occasionally to release the berries' natural juices. In a small bowl, whisk together the cornstarch and water to create a smooth slurry. Slowly pour the slurry into the berry mixture, stirring continuously to prevent clumping.

Continue cooking the mixture over medium heat, stirring frequently, until it reaches a gentle boil and thickens to a jam-like consistency. If the mixture remains too thin, prepare an additional small amount of cornstarch slurry and stir it in as needed. Remove from heat and mix in the lemon juice to enhance the flavor. Allow the filling to cool slightly before assembling the pie.

Pour the cooled elderberry filling into the prepared pie crust, spreading it evenly. Dot the filling with butter, which will add richness to the flavor. Roll out the top crust and place it over the filling. Seal the edges by pressing them together with your fingers or a fork. For a decorative touch, create a lattice crust by weaving strips of dough over the filling. Cut a few slits in the top crust to allow steam to escape.

Transfer the assembled pie to the preheated oven and bake for 40 minutes, or until the crust is golden brown and the filling is bubbling through the openings. If the edges brown too quickly, cover them with aluminum foil during the last 15 minutes of baking.

Remove the pie from the oven and allow it to cool on a wire rack for at least 2 hours. This cooling period is essential for the filling to set properly. Serve at room temperature or slightly warmed, optionally garnished with whipped cream or a scoop of vanilla ice cream.