With the rack in the middle position, preheat the oven to 350°F (180°C). Line a 17 x 12-inch (43 x 30 cm) baking sheet with parchment paper, letting it hang over both sides. Butter the other two sides.
In a bowl, combine the flour, baking powder and salt. Set aside.
In another bowl, beat the eggs, sugar and vanilla with an electric mixer until thick, triple in volume and forms a ribbon, about 10 minutes. Drizzle in the butter.
Sift the dry ingredients over the egg mixture and gently fold in with a spatula or whisk.
Spread the batter in the baking sheet. Bake for 15 minutes or until a toothpick inserted in the centre of the cake comes out clean. Let cool on a wire rack.