01 - Preheat oven to 350°F. Line a baking sheet with parchment paper.
02 - In a mixing bowl, cream together the softened butter, brown sugar, and granulated sugar until light and fluffy. Add the egg and beat until fully incorporated.
03 - Fold in the rolled oats, flour, baking powder, baking soda, and salt. Stir until just combined and no dry streaks remain.
04 - Spread the dough evenly across the prepared baking sheet into a thin, flat rectangle. Bake for 15 minutes or until the edges turn golden brown. Allow to cool completely on the sheet.
05 - Crumble the cooled oat cookie into fine crumbs. Add 3 tablespoons of melted butter and mix until the crumbs are evenly moistened. Press the mixture firmly into the bottom and up the sides of a 9-inch pie pan to form a uniform crust.
06 - Lower the oven temperature to 325°F.
07 - In a large bowl, whisk together the melted butter, granulated sugar, brown sugar, milk powder, flour, and salt until smooth. Add the egg yolks, whole egg, and vanilla extract, mixing until just combined — do not overmix.
08 - Pour the filling into the prepared crust. Bake for 30 to 35 minutes, until the edges are set and the center still has a slight jiggle.
09 - Transfer the pie to a wire rack and cool completely. Refrigerate for at least 2 hours, preferably overnight, to allow the filling to set properly.
10 - Dust the surface generously with powdered sugar just before serving. Slice into wedges and serve chilled.