Heat oven to 350. Spray a 12-cup bundt pan w/ cooking spray. In bowl, beat butter and 3/4 cup peanut butter until smooth. Add eggs and 1/2 cup buttermilk, beat until combined.
Add flours, sugar, water baking powder and vanilla. Beat on med-high until smooth. Pour into pan. Bake 45 mins, reduce temp to 325 and bake another 5-10 mins, or untill a toothpick inserted in center comes out clean.
Let cake cool, turn out onto a cooling rack. In bowl, combine powdered sugar, peanuts, remaining peanut butter and buttermilk. Mix well. Spoon over cake.