Preheat oven to 350°F/177°C.
Prepare 2 round 4" cake pans by lining the bottoms with parchment paper. (Note: You can use 3 round 4" cake pans if you want a taller cake, or use the remaining batter for cupcakes).
Separate the egg yolks from the egg whites (while cold), and place the egg yolks into a large bowl. Set aside.
Transfer the egg whites to a bowl of a stand mixer fitted with a wire whisk attachment.
Let the egg whites come to room temperature.
Begin whipping the egg whites on low speed, until foamy and bubbles appear.
Add in the cream of tartar and continue beating, until more bubbles appear.
Gradually pour in the remaining 25g granulated sugar and increase the speed.
Whip until the egg white meringue reaches soft peaks.
To the egg yolks, add in the milk, vegetable oil, 25g sugar, vanilla extract, and whisk until combined.
Sift in the cake flour and mix until incorporated (don't overmix).
Fold in about ⅓rd of the egg white meringue into the egg yolk cake batter to lighten the mixture.
Continue to fold the remaining egg white meringue mixture into the cake batter, until incorporated.
Divide batter evenly into the prepared pans.
Give the pans a tap to release any trapped air bubbles.
Bake at 350°F/177°C for 25-30 minutes, or until the tops are browned and springy to the touch.
Remove the cakes from the oven and let completely cool on a wire rack.