Preheat oven to 350°F Butter and flour three 9-inch-diameter cake pans. Line the bottoms with parchment paper, then butter and flour the parchment paper. (Or, line cupcake pans with paper liners).
In large bowl, cream together butter and sugar with electric mixer. Add peanut butter and vanilla and cream well. Add eggs one at a time, mixing well after each.
In separate bowl, whisk together dry ingredients.
Add dry ingredients to peanut butter mixture alternately with sour cream and milk. Mix on low speed just until blended.
Pour cake batter into prepared pans (approximately 2 cups per pan), spreading evenly in pans. (Or, fill cupcake cups 2/3 full).
Bake for 15- 20 minutes or until tester inserted into center of cake comes out clean. Do not over bake. Remove from oven and cool cakes in pans on rack 10 minutes.
Run small sharp knife around pan sides to loosen cakes. Turn cakes out onto racks and cool completely.
In large mixing bowl, beat butter until creamy. Mix in maple syrup and vanilla. Gradually mix in powdered sugar until desired consistency is reached.