With oven rack in center of oven, pre-heat oven to 350°F Line a twelve 2 1/2-inch muffin pan with paper liners and set aside.
In a large mixing bowl combine flour, baking powder, salt, and baking soda. Set aside.
In a large mixing bowl on medium speed beat margarine and oil together until well combined. Slowly add granulated sugar; beat until light and creamy. Add one large egg, then egg white, combining well after each addition. Blend in lemon zest.
Fold flour mixture and buttermilk alternately into egg mixture. Begin and end with the flour mixture. Stir in poppy seeds.
Spoon batter into prepared muffin pan. Bake cupcakes until golden brown approximately 20 to 22 minutes, or until a toothpick inserted in the middle comes out clean. When cool enough to handle remove from pans and finish cooling on a wire rack. These cupcakes can also be made ahead. Cool and wrap cupcakes well in plastic wrap, place in resealable plastic storage/freezer bags and freeze up to 1 month. Before adding lemon glaze,thaw cupcakes.
Lemon glaze: In a small mixing bowl ombine confectioners' sugar and lemon juice and stir until smooth. Drizzle or spread over cooled cupcakes.