9tablespoons(1 stick plus 1 tablespoon or 127 g) unsalted butter, room temperature, divided
5 ½Tablespoons(70 g) packed light brown sugar, divided
2Tablespoons(25 g) granulated sugar
1large egg, room temperature
¾cupplus 2 tablespoons old-fashioned oats
½cup(62 g) all purpose flour
⅛teaspoonbaking powder
⅛teaspoonbaking soda
¼teaspoongenerous salt
Filling
¾cup(150 g) granulated sugar
½cup(100 g) packed light brown sugar
1tablespoonnonfat dry milk powder
¼teaspoonsalt
½cup(1 stick or 113 g) unsalted butter, melted, cooled slightly
6 ½tablespoonsheavy whipping cream
4large egg yolks
1teaspoonvanilla extract
Powdered sugar, for dusting
Instructions
Oat Cookie Crust
Preheat oven to 350°F. Line a 13x9 cookie sheet (rimmed) with parchment paper and coat with nonstick spray. Set aside.
In the bowl of a stand mixer (or using a hand mixer), combine 6 Tablespoons butter, 4 Tablespoons brown sugar, and 2 Tablespoons sugar. Beat mixture until light and fluffy, about 2 minutes. Scrape down the sides of the bowl as necessary. Add egg and beat until pale and fluffy, about 2 more minutes. Add oats, flour, baking powder, baking soda, and salt and beat on medium speed until well blended, about 1 minute.
Turn oat mixture out onto prepared baking pan; press out evenly to edges of pan. If the dough is sticking to your fingers, wet them with water and continue pressing oat mixture out evenly. Cookie layer should be about 1/4-inch thick.
Bake until light golden, about 12 to 14 minutes (don't overbake). Transfer baking pan to rack and cool cookie completely.
Using hands (or a food processor), crush oat cookie into small crumbles and place in a large bowl. Add 3 tablespoons butter and 1 1/2 tablespoons brown sugar. Rub in with fingertips until mixture is moist enough to stick together. Transfer cookie crust mixture to 9-inch diameter glass pie dish. Using fingers, press mixture evenly onto bottom and up sides of pie dish. Place pie dish with crust on rimmed baking sheet.
Filling
Position rack in center of oven and preheat to 350° F.
In the bowl of a stand mixer fitted with the whisk attachment (or you can whisk vigorously by hand), combine both sugars, milk powder, and salt in medium bowl to blend. Add melted butter and whisk until blended. Add cream, then egg yolks and vanilla, and whisk until well combined and mixture is pale in color.
Pour filling into crust. Bake pie at 350° F for 20 minutes (filling may begin to bubble). Reduce oven temperature to 325° F. Continue to bake pie until filling is brown in spots and set around edges but center still moves slightly when pie dish is gently shaken, about 15 to 20 minutes longer. Cool pie 2 hours in pie dish on rack. Chill uncovered overnight.
Sift powdered sugar lightly over top of pie. Cut pie into wedges and serve cold.
Notes
Make ahead 2 days ahead. Cover and keep chilled.Store covered in the refrigerator for up to 5 days.Freeze pie after it has been chilled. Cover in plastic wrap and aluminum foil to prevent freezer burn. Store in the freezer for 3 months, then defrost in the refrigerator overnight before serving.