Preheat oven to 350 degrees F. Lightly grease a standard 12-well muffin pan or a jumbo 6-well muffin pan.
Sift together the flour, sugar, baking powder, baking soda, and salt into a large mixing bowl. In a separate bowl, whisk together the egg, buttermilk, and lemon juice.
Make a well in center of the flour mixture and pour the buttermilk mixture into the well. Add the melted butter and use a rubber spatula to blend with quick strokes, being careful not overmix the batter; it should still be slightly lumpy. Gently stir in the poppy seeds and lemon zest.
Spoon batter into the prepared muffin pan, filling each well 3/4 full. Bake until slightly puffed and golden, 20 to 22 minutes. Cooking time may be a little longer if you are baking jumbo-sized muffins. Lightly sprinkle sugar over the tops of the muffins, if desired. Remove the muffins from the pan and cool slightly on a wire rack. Serve the muffins warm, with Raspberry Butter.
Yield: 12 regular muffins, or 6 jumbo muffins