Cake:
1. COMBINE flour, baking powder, salt and first amount of sugar in large bowl. Stir well to blend.
2. ADD oil, egg yolks, water and lemon rind. Beat with electric mixer until smooth, about 30 seconds.
3. BEAT in small bowl egg whites and cream of tartar to form stiff, moist peaks. Gradually add sugar, beating until stiff, shiny peaks are formed.
4. FOLD egg whites into egg yolk mixture, gently but thoroughly.
5. TURN batter into ungreased 10" (4 L) tube pan.
6. BAKE at 350ºF (180ºC) for 60 minutes, or until toothpick inserted in centre comes out clean. Invert cake to cool. Loosen edges and shake pan to remove cake.
Filling:
7. CREAM butter until smooth. Gradually add half of icing sugar.
8. BEAT in lemon juice, egg yolk and pie filling, then remaining icing sugar gradually beating until smooth and creamy. Chill if necessary for desired consistency.
Assembly:
9. SLICE cake horizontally into 3 equal layers. Fill layers with 1/3 of filling. Spread remaining filling on top layer.
10. DECORATE top with lemon twists.