In a stand mixer, beat butter and sugar for 5-6 minutes until light.
Gradually add eggs and vanilla paste, mixing until incorporated.
Add flour, cornstarch, salt, then buttermilk; mix well.
If using homemade crust, roll out one disk into the pie plate. Fork-poke the crust, line with parchment, and fill with pie weights. Pre-bake for 10 minutes, then remove weights and parchment. Optionally, brush crust with egg wash (1 egg whisked with 1 tsp water).
Pour filling into crust. Bake at 400°F for 10 minutes, then reduce to 350°F and bake for 60-70 minutes. Turn off oven; let pie sit inside for 30 minutes until golden brown and knife comes out clean. Cover crust with shield or foil if browning too fast.
Notes
It is essential that all of your ingredients are room temp so that the pie sets up properly.