Preheat the oven to 375 degrees F (190 degrees C).

Line a 9-inch pie plate with 1 pie crust. Set aside.

Combine elderberries and sugar in a large pot. Whisk cornstarch and water together in a small bowl; stir into berries and cook over medium heat, stirring constantly, until filling bubbles and reaches desired thickness. Stir in more cornstarch slurry if filling is not thick enough. Off heat, stir in lemon juice.

Meanwhile, roll remaining crust into a 10-inch circle. Cut into 1/2-inch strips with a sharp paring knife or pastry wheel. Pour elderberry filling into the prepared pie crust; dot with butter. Moisten rim of pie with a small amount of water. Start with the longest strips and lay the first 2 in an X in the center of the pie. Alternate horizontal and vertical strips, weaving them in an over-and-under pattern. Use the shortest strips for the edges of the lattice. Fold the ends of the lattice strips under the edge of the bottom crust and flute the crust.

Bake in the preheated oven until crust is browned and filling is bubbling through holes in crust, about 40 minutes.