Start by preparing the egg whites. Preheat your oven to 350°F (175°C). Grease an 8-inch round cake pan and line the bottom with parchment paper. In a clean bowl, beat the egg whites together with the cream of tartar until they form soft peaks. Gradually add 1/2 cup of sugar while continuing to beat until the whites hold stiff peaks.
Next, work on the yolks and dry ingredients. In a separate bowl, whisk the egg yolks together with the remaining 1/4 cup of sugar until the mixture turns pale and slightly thickened. Stir in the flour, lemon zest, and salt until everything is fully combined.
Prepare the custard base by warming the milk in a small saucepan over low heat until it is lukewarm. Slowly whisk the warm milk into the egg yolk mixture, then mix in the melted butter, lemon juice, and vanilla extract.
Now, combine the two mixtures carefully. Fold the beaten egg whites into the yolk mixture in batches, using a gentle motion to keep the batter light and airy. Make sure not to deflate the egg whites.
Pour the batter into the prepared cake pan and smooth the top. Bake in the preheated oven for 45 to 50 minutes, or until a toothpick inserted into the center comes out clean. Let the cake cool in the pan for about 10 minutes before transferring it to a wire rack to cool completely.
Once cooled, dust the cake with powdered sugar. Slice and enjoy. For an extra touch, you can serve it with whipped cream or fresh berries.