Combine flour, baking powder, soda and salt in bowl and toss with fork to blend thoroughly. In large bowl of electric mixer, beat olive oil with eggs, pumpkin pie mix and 1/2 cup evaporated milk. Gradually beat in dry ingredients. Fold in chopped walnuts. Transfer to oiled fluted tube pan. Bake at 325 degrees for 45 minutes or until cake tests done. Cool on rack for 15 minutes then turn cake out onto rack and cool completely.
In small bowl, mix sugar, 4 tsps. remaining milk and maple flavoring. Stir until smooth. Add more milk, 1/2 tsp. at a time, if needed. Glaze should have a thick consistency. Drop spoonfuls of glaze evenly around the top of the cake. Set aside to allow glaze to drip down the sides of the cake. When glaze has set, garnish with walnut halves.