Step 1

Preheat oven to 350°F. Line a 13x9 cookie sheet (rimmed) with parchment paper and coat with nonstick spray. Set aside.

Step 2

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.

Step 3

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 3/8-inch thick.

Step 4

Bake until light golden on top, 12 to 14 minutes. Transfer baking pan to rack and cool cookie completely.

Step 5

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.

Step 6

Position rack in center of oven and preheat to 350° F.

Step 7

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.

Step 8

Pour filling into crust. Bake pie at 350° F for 30 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 20 minutes longer. Cool pie 2 hours in pie dish on rack. Chill uncovered overnight.

Step 9

Sift powdered sugar lightly over top of pie. Cut pie into wedges and serve cold.