Here's a recipe for a classic 9-inch sponge cake:

Ingredients:

• 4 eggs, at room temperature
• 1 cup granulated sugar
• 1 cup all-purpose flour
• 1 teaspoon baking powder
• 1/4 teaspoon salt
• 1/4 cup water
• 1 teaspoon vanilla extract
• Powdered sugar for dusting (optional)

Instructions:

1. Preheat your oven to 350°F (180°C). Grease and flour a 9-inch round cake pan and set aside.
2. In a large bowl, beat the eggs and sugar together with an electric mixer until light and fluffy, about 5-7 minutes.
3. In a separate bowl, sift together the flour, baking powder, and salt.
4. Add the dry ingredients to the egg mixture in three parts, folding gently with a spatula after each addition until just combined.
5. In a small bowl, combine the water and vanilla extract. Add this mixture to the batter and gently fold until just combined.
6. Pour the batter into the prepared cake pan and smooth the top with a spatula.
7. Bake for 25-30 minutes, or until the cake is golden brown and a toothpick inserted in the center comes out clean.
8. Remove the cake from the oven and let it cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.
9. Dust with powdered sugar, if desired, before serving.

Tips:

• Be careful not to overmix the batter or the cake will be tough.
• Make sure the eggs are at room temperature before beginning to beat them, as cold eggs will not whip up as well.
• For a fluffier cake, you can separate the egg whites from the yolks and beat the egg whites until stiff peaks form, then fold them gently into the batter at the end.
• This sponge cake can be served plain, or you can decorate it with whipped cream, fresh fruit, or frosting.