01 - Preheat oven to 350°F (175°C) and line a baking sheet with parchment paper.
02 - Using a mixing bowl and an electric mixer or whisk, blend softened butter, brown sugar, and granulated sugar until light and fluffy. Add the egg and mix thoroughly.
03 - Fold in rolled oats, flour, baking powder, baking soda, and salt until a cohesive dough forms.
04 - Spread dough evenly on prepared baking sheet in a single layer. Bake for 15 minutes or until golden brown. Allow to cool completely.
05 - Crumble cooled oat cookie into fine crumbs. Incorporate 3 tablespoons melted butter, mixing until evenly moistened. Transfer mixture to a 9-inch pie pan and press firmly across bottom and up sides to form the crust.
06 - Lower oven temperature to 325°F (160°C).
07 - Whisk together melted butter, granulated sugar, brown sugar, milk powder, flour, and salt until smooth. Add egg yolks, whole egg, and vanilla extract; gently mix until just incorporated, avoiding over-mixing.
08 - Pour prepared filling into oat cookie crust, spreading evenly.
09 - Bake for 30 to 35 minutes or until the edges are set but the center remains slightly wobbly.
10 - Cool pie completely on a wire rack. Refrigerate for at least 2 hours, preferably overnight, to allow filling to set.
11 - Dust the chilled pie generously with powdered sugar immediately before serving.