Preheat oven to 350ºF. Grease an 8-inch springform pan and line the bottom with a parchment paper round; set aside. In a mixing bowl, add flour, baking powder and salt; whisk until blended.
In another bowl, add egg whites and cream of tartar. Using a hand mixer, process until soft peaks form before gradually adding ½ cup sugar. Continue mixing until stiff peaks form.
In the bowl of a stand mixer, add the remaining ½ cup sugar, butter, egg yolks and vanilla extract. Using the paddle attachment, process on medium speed until the ingredients are mixed.
Add half dry ingredients and half milk. Process on low speed until blended before adding the remaining half dry ingredients and half milk. Process and clean the sides and the bottom of the bowl as needed.
Spoon meringue into the batter and using a spatula, gently fold it in – do not overdo it.
Pour the batter into the prepared springform pan and level it out – DON’T tap the pan on the counter.
Transfer to the preheated oven and bake for 40 to 45 minutes or until a cake tester inserted in the center comes out clean with just a few crumbs.
Remove from the heat to a wire rack and carefully unlatch the springform pan. Allow the cake to cool completely.