Preheat oven to 425°F (218°C). Spray 12-count muffin pan with nonstick spray. Set aside.

In a large bowl, whisk the flour, granulated sugar, poppy seeds, baking powder, baking soda, and salt together until thoroughly mixed. Set aside.

In a medium bowl, whisk the melted butter, lemon juice, lemon zest, orange juice, and orange zest together until combined. Add the eggs, one at a time, whisking after each addition. Whisk in the yogurt, milk, and vanilla. Pour the wet ingredients into the dry ingredients and gently mix together until no pockets of flour remain. Do not over-mix. The batter is thick.

Spoon the thick batter into the muffin tins, filling them all the way to the top. Sprinkle each with coarse sugar. Bake for 5 minutes at 425°F (218°C). Keeping the muffins in the oven, reduce oven temperature to 375°F (190°C) and continue to bake for 10-13 minutes longer until tops are lightly golden. A toothpick inserted in the center should come out clean. Allow to cool for 10 minutes. Top with additional lemon and/or orange zest if desired (optional).

Muffins stay fresh in an airtight container at room temperature for up to 5 days. Try them with a simple lemon glaze too!