- In a small bowl, dissolve yeast in warm milk. In a large bowl, combine 3 cups flour, 1 teaspoon sugar and salt. Cut in 1.5 cup butter until crumbly.
- Add yeast mixture and egg yolks; mix well. Stir in enough remaining flour to form soft dough. Combine walnuts and remaining sugar; set aside.
- Divide dough into thirds. Between waxed paper, roll one portion into a 15-in. x 10-in. rectangle. Transfer to a greased 15-in. x 10-in. baking pan. Sprinkle with 1-1/2 cups walnut mixture. Roll out second portion of dough; place over walnut layer. Spread preserve to within 1 in. of edges. Roll out remaining portion; place over filling.
- Bake at 325°F for 45-50 minutes or until golden brown. Cool completely on a wire rack. In a large bowl, beat the powder sugar, 6 oz butter, vanilla and enough milk to achieve desired consistency.
- Spread over top; sprinkle with remaining walnut mixture. Cut into 1-in. squares. Refrigerate leftovers.