Preheat an oven to 175°c/155°c fan (347°f/311°f). You'll need a 9"x13" baking tin for the cake but we'll be leaving this un greased & un lined.
Next, add the flour, baking powder & salt into a mixing bowl then whisk to combine. Set this aside for now.
Separate the eggs then place the whites into the bowl of a stand mixer & the yolks into a large mixing bowl.If you haven't got a stand mixer, place the whites into another large mixing bowl then use a hand mixer to whisk them (make sure to clean the beaters!). I'd recommend weighing out the whites & yolks. From 6 large eggs, you'll need 200 grams of whites & 100 grams of yolks. Add 60 grams of the caster sugar into the egg yolks then whisk with an electric hand mixer for 3-4 minutes, until pale & fluffy. Continue whisking the yolks (but on a low speed) then gradually pour in the milk, mixing until combined. Set this to one side.
Next, whisk the egg whites on a medium speed for 2-3 minutes, until frothy. Turn the mixer down slightly. add in the cream of tartar then gradually add in the remaining 200 grams of caster sugar a tablespoon at a time, making sure to let each addition incorporate fully before adding in more.
Once all the sugar has been added, turn the mixer back up to a medium speed then continue whisking until the whites reach stiff peaks. This will take another 3-4 minutes.
Sift half of the dry ingredients into the egg yolks then whisk gently by hand, until just incorporated.
Next, switch to a spatula then add half of the egg whites into the yolks then fold gently until just combined. Sift in the remaining dry ingredients, fold to combine then fold in the remaining egg whites.Make sure not to over mix the batter at this point! Transfer the cake batter into the prepared tin, gently spread out into an even layer then give the tin 1 firm tap on your counter, to pop any large air bubbles.
Bake the cake in the preheated oven for 25-30 minutes, until golden brown. If you gently press the top of the cake with a fingertip, it should feel bouncy & spring back.
Once cooked, let the cake cool at room temperature, until it is slightly warm. This will take around half an hour.The cake will shrink slightly as it cools, this is normal!