In a bowl, cream shortening and sugars. Beat in the egg, egg yolk and vanilla. Combine the flour, baking powder, salt and baking soda. Add to creamed mixture alternately with milk. Beat well. Stir in chocolate chips. Pour into a greased and waxed paper-lined 9-in. round baking pan. Bake at 350° for 30-35 minutes or until a toothpick inserted in the center comes out clean. Cool for 5 minutes before removing from the pan to a wire rack to cool completely.
Meanwhile, for filling, combine brown sugar and flour in a small saucepan; stir in milk. Bring to a boil; cook and stir for 3 minutes or until thickened. Remove from the hear; stir in nuts, butter and vanilla; cool completely.
Cut cake widthwise in half. Place one layer on a serving plate. Spread with filling; top with other layer. In a microwave or small saucepan, melt chocolate chips and butter with corn syrup, stirring until smooth. Cool slightly and drizzle over top of cake.