01 - Preheat oven to 350°F. Line a baking sheet with parchment paper.
02 - Cream together butter, brown sugar, and granulated sugar until light and fluffy. Add egg and mix well.
03 - Stir in oats, flour, baking powder, baking soda, and salt. Mix until just combined.
04 - Spread dough evenly onto prepared baking sheet in a thin rectangle. Bake for 15 minutes until golden. Allow to cool completely.
05 - Crumble cooled oat cookie into fine crumbs. Mix with 3 tablespoons melted butter to form moist mixture. Press evenly into bottom and up sides of a 9-inch pie pan.
06 - Lower oven temperature to 325°F.
07 - Whisk together melted butter, granulated sugar, brown sugar, milk powder, flour, and salt. Add egg yolks, whole egg, and vanilla. Mix until just combined.
08 - Pour filling into prepared crust. Bake for 30-35 minutes until edges are set and center remains slightly wobbly.
09 - Cool completely on wire rack, then refrigerate for at least 2 hours (preferably overnight) before serving.
10 - Dust generously with powdered sugar before serving.