In a large bowl of an electric mixer cream the butter, sugar, and zest together until fluffy and pale. Add the eggs, one at a time, beating well after each addition.

Sift flour, buttermilk powder (if using), baking powder, and salt into a bowl, and mix in the poppy seeds (if using).

Add the flour mixture to the cream mixture in three additions, alternating with the milk mixture in two additions, beginning and ending with the dry ingredients. Don’t overmix, just fold gently; the batter may appear curdled, but don’t worry

Scrape batter into the prepared pan, smoothing the top. Bake for 1 hour to 1 hour and 15 minutes, or until a tester inserted in the center comes out clean. Transfer pan to a wire rack and cool for 10 minutes.

For the glaze, in a medium-sized bowl, cream the butter until light and fluffy. Gradually beat in the sifted sugar and the combined zest and lemon juice, alternating one and then the other, until creamy, pourable consistency is achieved.

Invert cake onto a wire rack set over a baking sheet. Spoon glaze over hot cake and allow to cool completely before cutting. Individually wrapped slices will freeze well.