Heat oven to 325°F. Grease bottom and side of 12-cup fluted tube cake pan with shortening; lightly flour. Sprinkle nuts evenly in bottom of pan. In medium bowl, mix flour, sugar, baking powder and salt; set aside. In large bowl, beat 1/2 cup butter and 3/4 cup peanut butter with electric mixer on medium speed until smooth, scraping bowl occasionally. Beat in eggs. On low speed, beat in milk and vanilla. Beat in flour mixture just until blended, scraping bowl occasionally. Spread in pan.