Yield: one 9-inch pie
Prepare pie dough.
Roll dough on a lightly floured work surface. Roll to about 10-inches in diameter and about 1/4-inch in thickness. Place pie plate upside down over the rolled dough to make sure it is the correct size.
Transfer dough to pie plate and firmly press into the bottom and sides of plate. Trim excess dough leaving about 1/2-inch overhang all around. Fold overhang into the pie and flute the edges with a fork, or crimp with your fingers. Cover pie dough with plastic wrap and let rest in the refrigerator while you prepare filling.
Preheat oven to 350 degrees F. In a large bowl, whisk together eggs, yolk and brown sugar until smooth. Add buttermilk, melted butter, flour, vanilla, and cinnamon. Whisk until combined. Pour mixture into prepared pie shell.
Bake for 40-50 minutes until center of pie is set and no longer jiggly. Remove from oven and allow to cool to room temperature on wire rack. Alternatively, store pie in fridge to cool faster. Serve with whipped cream.
Pulse together flour, salt, and sugar in a food processor. Add cold butter a couple cubes at a time with the food processor running. Stop the machine once mixture is crumbly and like coarse sand in texture.
Add 2 tablespoons of water and yolk. Pulse for a couple seconds until dough begins to come together. If dough is too dry, add another tablespoon of water. Remove dough from machine and finish kneading on a lightly floured surface.
Flatten dough into a disk and wrap in plastic wrap. Chill in the refrigerator for 20 minutes or freeze for 10 minutes.