Coat an 8-inch round cake pan with cooking spray. Line bottom of pan with a round of wax paper; coat wax paper and sides of pan with cooking spray.
Combine milk and butter in a small saucepan; place over medium heat 4 minutes until mixture almost comes to a simmer. Remove from heat, and set aside.
Beat egg whites in a large bowl until soft mounds form. Gradually add sugar, beating until stiff peaks form. Beat in egg yolks.
Combine flour and baking powder in a small bowl. Add half of flour mixture to egg mixture; beat briefly. Add hot milk mixture and vanilla to egg mixture; beat until thoroughly combined. Beat in remaining flour mixture and 3 tablespoons of hazelnuts. Batter will be thin. Pour into prepared pan, and sprinkle remaining 1 tablespoon hazelnuts over batter. Bake at 350° for 23 minutes until top springs back when lightly touched. Cool in pan on a wire rack 10 minutes. Run a knife around sides and unmold cake. Peel off wax paper, and place cake, right side up, on a rack. Let cool completely.
Split cake in half horizontally, using a serrated bread knife. Spread chilled Pastry Cream over bottom portion. Add top cake layer, and chill 1 hour. Remove from refrigerator 20 minutes before serving. Sift 1 teaspoon powdered sugar over top.