Preheat the oven to 350°F (180°C)
Butter or spray an 8-inch square baking pan lined with parchment paper.
Mix the dry ingredients – flour, salt and baking powder and give it a whisk or stir. Leave it aside.
Melt butter either in microwave or stove top in a pan or butter warmer. Let it cool slightly, but still liquidy add both sugar (granulated white sugar and confectioners’ sugar) one by one and whisk or if you are using a mixer, let it mix for 1 minute.
Add eggs and vanilla extract, followed by lemon juice and zest. Whisk everything until it looks homogenous.
Add the dry ingredients into wet mix and using a spatula, mix everything well. The batter will be thick. Do not over mix it, make sure dry ingredients are mixed well.
Transfer batter to prepared baking pan. Make sure to even out the batter in the pan.
Bake for 15 minutes and at 350 F.