In a large bowl, cream butter and sugar until light and fluffy.
Add eggs, one at a time, beating well after each addition
.Beat in vanilla.Combine the flour, baking soda and salt; add to creamed mixture alternately with sour cream, beating just until combined.
Pour into two greased and waxed paper-lined 9-in. round baking pans.Combine topping ingredients; sprinkle over batter.
Lightly cut through with a knife to swirl.Bake at 350° for 20-25 minutes or until a toothpick inserted near the center comes out clean.
Cool for 10 minutes; remove from pans to wire racks to cool completely.In a small saucepan, combine cornstarch and milk until smooth.Bring to a boil; cook and stir for 1-2 minutes or until thickened.
Cover and refrigerate until chilled.In a small bowl, cream the butter, shortening and sugar until light and fluffy.
Add vanilla and chilled milk mixture; beat vigorously by hand or on medium speed with a mixteruntil smooth and creamy, about 10 minutes.
Place one cake on a serving plate; spread with filling.Top with remaining cake.Store in the refrigerator or cool cellar.Yield: 8-10 serving
