1Arrange oven rack in center of oven. Heat oven to 350 degrees F. Line twelve 2 1/2 inch muffin cups with paper liners.
2Stir flour, baking powder, salt, and baking soda together in a large bowl until well combined.
3Beat margarine and oil together in a large mixer bowl on medium speed until combined. Gradually add granulated sugar; beat until light and creamy. Add whole egg, then egg white, beating well after each addition. Beat in lemon zest.
4Alternately fold flour mixture and buttermilk into egg mixture, beginning and ending with flour mixture. Stir in poppy seeds.
5Spoon batter into muffin cups. Bake until golden brown and a toothpick inserted into center of cupcakes comes out clean, 20 to 22 minutes. Remove from pans and cool on a wire rack. (Can be made ahead. Cool, wrap cupcakes well in plastic wrap, place in resealable plastic storage bag and freeze up to 1 month. Thaw before proceeding.
6Make lemon glaze. Combine confectioners' sugar and lemon juice in a small bowl, stirring until smooth. Drizzle or spread over cooled cupcakes.