Preheat oven to 350°F. Line a 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. Using electric mixer, beat mixture until light and fluffy, occasionally scraping down sides of bowl, about 2 minutes.

Add egg; beat until pale and fluffy. Add oats, flour, baking powder, baking soda, and salt and beat until well blended, about 1 minute. Turn oat mixture out onto prepared baking pan; press out evenly to edges of pan.

Bake until light golden on top, 17 to 18 minutes. Transfer baking pan to rack; let cool 5 minutes and then turn out cookie on parchment and let cool completely.

Using hands, crumble oat cookie into large bowl; add the 4 Tbsps butter and 1 & ½ Tbsps brown sugar. Rub in with fingertips until mixture is moist enough to stick together.

Line 9X9" baking pan with parchment paper, leave overhang for easy removal. Transfer cookie crust mixture to pan. Using fingers, press mixture evenly onto bottom. Place dish with crust on rimmed baking sheet.

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

Whisk 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 blended. Pour filling into crust. Bake for 30 minutes (filling may begin to bubble).

Reduce oven temperature to 325°F. Continue to bake until filling is brown in spots and set around edges but center still moves slightly when dish is gently shaken, about 15-20 minutes longer.

Cool in pan for 2 hours on rack. Chill uncovered overnight.

Sift powdered sugar lightly over top of dessert. Carefully remove from pan using parchment paper and cut into bars, serve cold.