Coat an 8-inch baking pan with nonstick cooking spray.
Topping: In a medium bowl stir together flour, sugars, pumpkin pie spice and salt. With a pastry cutter,cut butter into the mixture. Stir in walnuts. Refrigerate.
Cake: In a large bowl, whisk together the flour, baking powder, baking soda, pumpking pie spice and salt. Set aside.
In another large bowl, beat butter and sugars on medium speed until creamy and blended, about 3 minutes. Beat in eggs, one at a time. Add vanilla and beat until smooth.
On low speed, beat in the flour mixture in three additions, alternating with buttermilk. Beat for 2 minutes.
Spoon half of the batter into the prepared pan. Sprinkle half of the topping over the batter. Spoon remaining batter over topping. Sprinkle remaining topping over top of cake.
Bake at 350 degrees F for 50 to 60 minutes or until toothpick inserted in center of cake comes out clean. Cool completely on wire rack. Loosen edges with a knife or small spatula. Cut into 12 equal pieces.
In a small bowl, mix together confectioners' sugar and just enough milk to get a good drizzling consistency. Drizzle over cake squares and allow to set before serving.