Preheat oven to 350°F. Line 13x9x2-inch metal baking pan with parchment paper; coat with nonstick spray.
Combine 6 tablespoons butter, 4 tablespoons brown sugar, and 2 tablespoons sugar in medium bowl. Beat until light and fluffy. Add egg; beat until pale and fluffy. Add oats, flour, baking powder, baking soda, and salt and beat until well blended. Press out onto prepared baking pan and bake until light golden on top.
Crumble oat cookie into a large bowl; add butter and brown sugar. Rub in with fingertips until moist.
Press cookie crust mixture onto the bottom and up the sides of a pie dish. Place on a rimmed baking sheet.
Whisk both sugars, milk powder, and salt in a medium bowl. Add melted butter, cream, egg yolks, and vanilla. Whisk until well blended. Pour filling into crust.
Bake pie, then cool for 2 hours. Chill uncovered overnight. Dust with powdered sugar before serving.
