Begin by preheating your oven to 350°F (175°C). Lightly grease an 8×8-inch baking pan or line it with parchment paper for easy removal. In a large mixing bowl, cream together the softened butter and granulated sugar until the mixture is light and fluffy. This step is important—it helps incorporate air into the batter, giving your cake that soft, fluffy texture.

Next, beat in the egg whites one at a time, mixing until well combined. Add the vanilla extract and mix again. In a separate bowl, whisk together the flour, baking powder, and salt. Gradually add the dry ingredients to the wet mixture, alternating with the milk. Stir gently and avoid overmixing, as this can lead to a dense or chewy cake.

Pour the batter into your prepared pan, smoothing the top with a spatula. Bake for 25–30 minutes or until a toothpick inserted in the center comes out clean. Let the cake cool in the pan for 10 minutes, then transfer it to a wire rack to cool completely. Slice and serve plain, dusted with powdered sugar, or topped with fruit and whipped cream.