Preheat oven to 325 degrees. Spray an 8- by 8-inch square baking pan with cooking spray. In a medium bowl, cream butter and sugar. Add one egg yolk and vanilla and beat at medium speed until light and fluffy. Add flour and salt and beat at low speed just until combined. Pour dough into prepared baking pan and chill. When chilled, press into pan with floured fingers and press dough to 1/4 inch thick on bottom and sides of pan. Extend up 1 inch of pan and chill again.

Filling: Mix together sugar and cornstarch in small mixing bowl. With electric mixer on medium high, beat 6 yolks until light and fluffy. Add cornstarch-sugar mixture and mix on medium-high until combined. Add vanilla, cream, chocolate and walnuts and blend on low, scraping bowl, whenever necessary, until combined. Pour filling into baking pan. Bake 1 hour until filling is set and golden brown. Chill and cut into bars.