Sift together flour, baking powder, baking soda and salt. Set aside.
Cream together butter, sugar and brown sugar in bowl until light and fluffy, using an electric mixer at medium speed. Add eggs, one at a time, beating well after each addition. Blend in pumpkin and lemon extract (total beating time: 10 minutes).
Gradually add dry ingredients alternately with milk to creamed mixture, beating well after each addition, using low speed. Stir in walnuts. Pour batter into greased 10” (25cm) cake tin.
Bake in 160 degrees C oven for 1 hour 20 minutes or until a cake tester or toothpick inserted into the centre comes out clean. Cool in tin for 10 minutes. Removed from tin. Cool on rack.
Sprinkle with icing sugar.
Serve with custard, whipped cream or a scoop of ice cream.