Preheat oven to 350°. Spray 2 (6-cup) cast-iron muffin pans with baking spray with flour.
In a large bowl, whisk together flour, brown sugar, poppy seeds, baking powder, salt, and baking soda. In a medium bowl, whisk together buttermilk, melted butter, egg, vanilla, and 3 teaspoons lemon zest. Whisk buttermilk mixture into flour mixture until combined. Divide batter among prepared muffin cups.
In a small bowl, whisk together granulated sugar and remaining 1 teaspoon lemon zest; sprinkle mixture onto batter.
Bake until a wooden pick inserted in center comes out clean, 18 to 20 minutes. Let cool in pans for 10 minutes. Serve warm, or remove from pans, and let cool completely on a wire rack.
Recipe by Southern Cast Iron at https://southerncastiron.com/lemon-poppy-seed-muffins/