Preheat oven to 350° F. Grease and flour a 10-inch Bundt cake pan. Set aside.
In large mixing bowl, combine butter, peanut butter, brown sugar and vanilla. Beat until light and fluffy. Add eggs, one at a time, beating well after each addition.
In another medium bowl, combine flour, baking powder and kosher salt. Stir into butter mixture alternately with milk. Pour batter into prepared cake pan.
Bake for about 1 hour and 15 minutes, or until toothpick inserted in center comes out clean. Cool in pan for 15 minutes, then remove from pan and cool completely on wire rack.