Preheat the oven to 170℃ / 340℉. Line a 23cmx23cm / 9inchx9inch baking tin with cooking spray and baking paper. Set aside.
Whisk the eggs, sugar and vanilla essence together until pale in colour and bubbly.
Sift in half of the flour and gently fold in until fully combined. Sift in the remaining flour with baking powder and salt. Again, fold in until fully combined.
Add the melted butter and continue folding until combined.
Pour into your baking tin and bake for 25-30 minutes or until a toothpick/skewer comes out clean. Set aside and allow to reach room temperature. Once cooled, trim the edges and surface (if the sponge is rounded on top) and cut into equally sized squares. Aim for 4 squares per side, yielding 16 squares in total.