Cream butter until light and fluffy. Sift in flour, sugar, baking powder, and salt. Add peanut butter and 2/3 cup of the milk. Mix until flour is well moistened. Beat for 2 minutes on low speed of electric hand-held mixer. Beat in remaining 1/3 cup of milk, the eggs, and vanilla. Beat for 2 minutes. Grease and flour two 9-inch layer cake pans; pour batter into the pans. Bake at 375° for about 25 minutes, or until a wooden pick or cake tester inserted in center comes out clean. Cool, carefully remove from pans, frost, and assemble layers.