Preheat your oven to 350°F (175°C). Grease and lightly flour a 9-inch cake pan or line it with parchment paper to ensure your cake releases easily.
In a medium bowl, whisk together the all-purpose flour, salt, baking powder, and poppy seeds. Set aside.
Using your mixer, beat the softened butter with 1 cup of the granulated sugar until light and fluffy, about 3-4 minutes. This step is crucial for a tender crumb.
In a separate small bowl, combine 3/4 cup of milk and the vanilla extract.
Add the dry ingredients to the butter mixture in three parts, alternating with the milk mixture. Begin and end with the dry ingredients, mixing gently to combine without overworking the batter.
In a clean bowl, beat the egg whites until soft peaks form. Gradually add the remaining 3/4 cup sugar while continuing to beat until stiff peaks form. This gives the cake its light, airy texture.
Carefully fold the whipped egg whites into the batter in three additions, preserving as much air as possible to keep the cake fluffy.
Pour the batter into your prepared pan and smooth the top with a spatula. Bake for 40-45 minutes, or until a toothpick inserted in the center comes out clean.
Allow the cake to cool in the pan for 10 minutes before transferring to a wire rack. Once completely cooled, dust with powdered sugar if desired.
