Preheat the oven to 350°F (175°C).

Roll out the uncooked pie crust to an even thickness.

Cut the pie dough into eighteen 4 1/2-inch circles.

Press one dough circle into each cupcake-tin cavity, smoothing the folds up the sides with your fingers.

Add the packed brown sugar, rolled oats, buttermilk, granulated sugar, eggs, butter, vanilla extract, and salt to a mixing bowl.

Whisk the filling until combined, leaving small pieces of room-temperature butter visible.

Spoon just under 3 tablespoons of oat filling into each prepared pie shell.

Bake the mini pies for 20 to 25 minutes, until golden brown and cracked on top.

Check that a knife inserted into the center comes out clean, even if it has a light oily coating.

Transfer the pies from the cupcake tins to a cooling rack and cool completely before serving.