In a large mixing bowl, beat the butter with an electric mixer for 30 seconds. Beat in sugar, baking powder, baking soda and salt until combined. Add eggs, pumpkin, sour cream, milk, and vanilla; beat until combined. Add all-purpose and whole wheat flours; beat until combined. Stir in 1 cup nuts.
Spread mixture evenly into prepared baking pan. Bake in a 350 degrees F oven about 25 minutes or until a wooden toothpick inserted near the center comes out clean. Cool in pan on wire rack.
Prepare Browned Butter Frosting; spread over cooled bars. If you like, sprinkle with additional nuts. Cut into bars. Makes 48 bars.
Browned Butter Frosting
In a small saucepan, heat butter over medium-low heat until melted. Continue heating until the butter turns a light brown. Remove from heat and transfer butter to a medium mixing bowl. Add powdered sugar, milk and vanilla. Beat with an electric mixer until combined. Beat in additional milk, 1 teaspoon at a time, to make a spreadable frosting. Use immediately. Makes about 1 cup.