01 - Preheat oven to 350°F. Line a baking sheet with parchment paper.
02 - In a mixing bowl, cream softened butter, light brown sugar, and granulated sugar until light and fluffy. Beat in the egg until fully incorporated.
03 - Fold in rolled oats, all-purpose flour, baking powder, baking soda, and salt. Mix until just combined, being careful not to overwork the dough.
04 - Spread the dough evenly across the prepared baking sheet into a thin, even layer. Bake for 15 minutes or until golden brown. Allow to cool completely on the sheet.
05 - Crumble the cooled oat cookie into fine crumbs. Toss with 3 tablespoons melted butter until the mixture resembles wet sand. Press firmly into the bottom and up the sides of a 9-inch pie pan to form an even crust.
06 - Lower oven temperature to 325°F.
07 - Whisk together melted butter, granulated sugar, light brown sugar, dry milk powder, all-purpose flour, and salt until smooth. Add egg yolks, the whole egg, and vanilla extract. Stir until just combined — avoid overmixing to prevent air bubbles.
08 - Pour the filling into the prepared oat cookie crust. Bake for 30–35 minutes, until the edges are firmly set and the center still has a slight wobble.
09 - Transfer the pie to a wire rack and cool completely. Refrigerate for at least 2 hours, preferably overnight, for the best texture and flavor development.
10 - Dust generously with powdered sugar just before serving. Slice into wedges and serve chilled.