Preheat oven to 400°F and line a 12-cup standard muffin pan with baking cups; spray baking cups lightly with cooking spray. Whisk together flour, poppy seeds, sugar, baking powder, baking soda, and salt in a large bowl. Make a well in the center of the mixture. Combine Meyer lemon zest, Meyer lemon juice, buttermilk, coconut oil, egg, and vanilla in a medium bowl; add to flour mixture and stir just until combined.
Fill each baking cup 3/4 full of batter and place pan in oven; immediately reduce oven temperature to 375°F and bake for 12 minutes, until muffins are golden and a toothpick inserted in the center comes with a few crumbs attached.
While the muffins bake, mix sugar and Meyer lemon zest in a small bowl; rub the zest into the sugar until with your fingertips until moist and fragrant. For the lemon syrup, combine Meyer lemon juice, sugar, and salt in a small saucepan over medium heat. Simmer, stirring occasionally, until sugar dissolves and syrup thickens, about 4 minutes.
Remove muffins from pan and transfer to a wire rack; let cool 5 minutes. Brush tops of muffins with lemon syrup, then dip muffins into lemon-sugar and serve. Store leftover muffins in an airtight container at room temperature overnight, or individually wrap muffins in plastic wrap and freeze in a large freezer bag.
