In a bowl combine flour, baking powder and salt. Set aside. In a large bowl, using an electric mixer, combine oil and sugar. Add eggs one at a time, beating well after each addition, then continue to beat 6 to 8 minutes on high speed until the mixture is pale and thick. Fold in vanilla, lemon juice and zests. Fold in dry ingredients alternating with milk and yogurt, beating on low speed after each addition until the batter is smooth but no more.
Spoon batter into two 9-inch buttered and floured round pans (or 24 lined muffin pans) . Bake in preheated oven at 350F for 30-35 minutes (or 18-20 minutes for cupcakes) or until a toothpick inserted in centers comes out clean. Cool on a wire rack for 10 minutes. Invert cakes onto rack and cool completely. Frost cake with buttercream frosting.
For Classic Vanilla Buttercream Frosting
Using an electric mixer with a paddle attachment on a medium setting, cream the butter until it is smooth and has lightened in color, about 3 minutes. Add 3 cups of confectioner's sugar, ½ a cup at a time. After each cup has been incorporated, turn the mixer onto the highest speed setting and for about 10 seconds to lighten the frosting. Fold in vanilla, salt and 2 tbsp of milk or other. Adjust with remaining sugar and milk. For a firmer frosting, add more confectioner's sugar, a ¼ cup at a time. For a softer frosting, add more milk or cream, a tablespoon at a time.