Step 1

In a large bowl, cream butter and sugar until light and fluffy, 5-7 minutes. Add eggs, 1 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.

Step 2

Pour into 2 greased and waxed paper–lined 9-in. round baking pans. Combine topping ingredients; sprinkle over batter. Lightly cut through with a knife to swirl.

Step 3

Bake at 350° until a toothpick inserted in the center comes out clean, 20-25 minutes. Cool for 10 minutes; remove from pans to wire racks to cool completely.

Step 4

For filling, 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, 5-7 minutes. Add vanilla and chilled milk mixture; beat on medium speed until smooth and creamy, about 10 minutes.

Step 5

Place 1 cake on a serving plate and spread with filling. Top with remaining cake. Store in the refrigerator. If desired, serve with caramel topping.