Preheat the oven to 160C or 150C fan-forced. Have your cake pan handy but DO NOT grease it - it must be left dry so that the cake clings to it when inverted.
In a large mixing bowl, combine the egg yolks and 200g sugar and whisk by hand until thick and creamy. Whisk in the oil and pandan liquid until combined.
Gently whisk in the flour, baking powder and salt.
Use an electric mixer to whisk the egg whites and cream of tartar (if using) at medium-high speed until soft peaks form.
Add the 50g sugar in two additions and continue whisking until stiff peaks form and the mixture is smooth and glossy.
Take a large spoonful of egg whites and mix this into the pandan mixture - you don't need to be gentle with this first amount of egg white as it is being 'sacrificed' to lighten the mixture, which will allow the rest of the egg whites to be incorporated more easily. Now gently fold in the remaining egg whites, using a balloon whisk, until fully incorporated.
Pour the mixture into the cake pan and tap the pan lightly on the bench to settle any bubbles.
Bake for 1 hour or until a skewer inserted into the cake comes out clean.
Meanwhile, prepare the coconut buttercream.
Remove the cake from the oven and immediately invert the cake either onto the bench (if your tin has feet) or onto the neck of a bottle. Allow the cake to cool completely before turning the right way up.
To remove the cake, run a knife or palette knife around the edge to separate it from the side of the pan, then run the knife or palette knife around the base to release the cake.
Place the cake on a serving plate, then use a palette knife to spread the coconut buttercream around the sides and top of the cake and garnish with toasted coconut.
