Preheat oven to 160℃.
Grease and line a 20cm (8") square baking pan with baking/parchment paper (leaving some overhang for removal) and set aside.
In a medium bowl, place plain flour and baking powder and whisk to combine, then set aside.
190 grams plain flour (all-purpose flour), ½ teaspoon baking powder
In a large bowl, combine caster sugar and grated lemon zest, and using a flexible spatula, rub the lemon zest into the sugar to release the oils.
150 grams caster sugar, zest of 1 lemon
Mix in the melted butter.
120 grams salted butter
Then add the lemon juice, vanilla and egg, and mix to combine.
1 tablespoon fresh lemon juice, 1 teaspoon vanilla extract, 1 egg
Add the flour mixture and mix until just combined.
Pour batter into the prepared pan, pushing it into the corners and roughly smoothing the top.
Evenly sprinkle over extra sugar, then bake for 19 - 22 minutes, or until the centre is just set, the edges are starting to lightly brown, and a toothpick inserted into the centre comes out clean with a few moist crumbs.
Allow to cool in the pan for 10 minutes, then remove and allow to cool completely on a wire rack before slicing.