Preheat the oven to 375 degrees F. Lightly brush a 12-cup muffin tin with butter and set aside. Sift the flour, baking powder, and salt into a medium bowl and set aside.

In a standing mixer fitted with the paddle attachment, or with an electric hand-held mixer in a large bowl, cream the butter, zest, and 2/3 cup sugar until light and fluffy, about 2 minutes. Scrape down the sides of the bowl with a rubber spatula. Add the eggs, one at a time, beating well after each addition. Remove the bowl from the mixer. Stir in the poppy seeds.

Fold the flour in 3 parts into the butter mixture, alternating with the milk in 2 parts, until just combined. Take care not to overmix the batter. Divide the batter evenly into the muffin tin and sprinkle the tops with sugar. Bake until golden brown, about 25 minutes. Cool muffins in the pan on a rack. Serve warm.

Cook's Note: Poppy seeds can go rancid easily and should be stored in the refrigerator or freezer.