Line one cupcake pan with 12 cupcake liners and a second pan with 3 liners. Preheat the oven to 325 F.
Process the sugar in a food processor or blender until fine and powdery. Then pulse in the lemon zest. Set aside.
Sift the flour into a bowl. Then whisk in ¼ cup of the processed lemon sugar, poppy seeds, and salt. Set aside.
In a mixing bowl, whisk the egg whites and lemon juice together on medium-low until foamy, about 1 minute. Increase the speed to medium-high and slowly sprinkle in the remaining ½ cup of sugar. Keep mixing until soft peaks form, about 5-6 minutes.
Sift ⅓ of the flour mixture into the bowl of egg whites and use a rubber spatula to gently fold it in. Continue this same process with the rest of the flour mixture.
Spoon or use a cookie scoop to fill the cupcake liners ⅔-3/4 full with the batter.
Bake the cupcakes for 18-20 minutes or until very lightly browned around the edges and when a toothpick inserted into the center comes out clean.
Allow the cupcakes to cool for 10 minutes in the pan on a wire rack, then remove them and place them directly on the rack to cool completely before frosting.