Preheat the oven to 350 degrees F. Coat a muffin tin with nonstick spray or line with paper liners. In a small bowl, melt the butter. Set aside to cool to room temperature.
In large bowl, whisk together the whole wheat flour, all-purpose flour, baking powder, baking soda, salt, and poppy seeds. Set aside.
Place the sugar in a medium bowl and add the lemon zest (zest the lemons directly over the bowl to capture their flavorful oils). With your fingers, rub the two together until the sugar is moist and fragrant. Add the yogurt, eggs, lemon juice, vanilla, and melted butter. Whisk until smooth.
Make a well in the center of the dry ingredients, then carefully pour in the wet ingredients. With a spatula or wooden spoon, mix gently just until the flour disappears. The batter will be thick.
With a scoop, divide the batter evenly between the muffin cups (they'll be 2/3 to 3/4 of the way full).
Bake lemon poppy seeds muffins for 18 to 20 minutes, or until toothpick inserted in the center of a muffin comes out clean. Let cool in the pan for 5 minutes, then carefully lift the muffins out of the pan and transfer them to a wire rack to cool completely (this will keep the muffins from steaming in the pan and getting soggy).
While the muffins cool, make the glaze: Place the lemon juice and powdered sugar in a small bowl and whisk briskly until smooth. Add more powdered sugar or lemon juice as needed so that your glaze is a pourable consistency. Drizzle over the cooled muffins.
Notes
Adapted from the Lemon Chia Seed Muffins in my cookbook.
TO STORE: Store muffins in a paper towel-lined airtight container at room temperature on counter for up to four days.
TO FREEZE: Freeze muffins in an airtight, freezer-safe storage container for up to three months. Thaw and enjoy as desired. I like to individually wrap the muffins first, so I can use them for quick breakfasts and snacks.