Preheat the oven to 350 degrees (F). Spray a 9-inch square cake pan with non-stick spray and lay 2 strips of parchment paper in a cross shape across the bottom. Set aside.
In a large bowl combine the granulated sugar and the lemon zest. Whisk well to help release the oils in the zest.
Whisk in the eggs, vanilla, sour cream, lemon juice, poppy seeds, and melted butter until combined.
In a separate bowl combine the flour, baking soda, baking powder, and salt. Fold the dry ingredients into the wet ingredients until just combined.
Pour the batter into the prepared pan and bake for 35-40 minutes or until an inserted toothpick comes out clean.
Remove from oven and allow to cool.