Preheat the oven to 350°F (175°C) and line a muffin tin with paper liners.

In a bowl, combine the flour, baking powder, baking soda, salt, and poppy seeds.

In another bowl, whisk together the sugar, melted butter, egg, buttermilk, lemon juice, and lemon zest until well combined.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

Fill the muffin cups about two-thirds full with the batter.

Bake in the preheated oven for 20 minutes, or until a toothpick comes out clean.

Let cool in the tin for a few minutes before transferring to a wire rack to cool completely.