Prepare Your Workspace: Preheat your oven to 375°F (190°C) and line a muffin tin with paper liners or lightly grease it with cooking spray.
Mix the Dry Ingredients: In a medium-sized mixing bowl, whisk together the flour, poppy seeds, baking powder, baking soda, and salt. Set aside.
Combine the Wet Ingredients: In a large bowl, whisk the melted butter and sugar until smooth. Add the eggs one at a time, whisking well after each addition. Stir in the Greek yogurt, milk, lemon zest, lemon juice, and vanilla extract until fully combined.
Combine Wet and Dry Ingredients: Gradually add the dry ingredients to the wet ingredients, folding gently with a spatula until just combined. Be careful not to overmix; a few lumps are okay.
Fill the Muffin Tin: Divide the batter evenly among the prepared muffin cups, filling each about two-thirds full.
Bake: Place the muffin tin in the preheated oven and bake for 18-20 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Cool and Serve: Allow the muffins to cool in the tin for about 5 minutes, then transfer them to a wire rack to cool completely. Serve warm or at room temperature.