• Preheat oven to 350 F. In a medium bowl, combine flour, salt and baking powder. In the bowl of a stand mixer, whisk the egg whites on medium-high speed until frothy. Add the cream of tartar and continue to whisk until egg whites form soft peaks. Gradually add 1/2 cup sugar while the mixer is running and continue beating until the egg whites form firm peaks that hold its shape when the whisk is lifted.
• Meanwhile in another bowl, beat the egg yolks in a bowl until it is slightly thicker. Add the remaining 1 cup sugar and mix until incorporated. Add water, oil and pandan extract and mix well. 
• Add flour mixture gradually and mix until the batter is smooth. Gently fold the egg whites to the yolk batter in three additions until batter is uniform. Pour batter into a prepared 10 inch tube pan. Bake for 60-65 minutes, or until a toothpick inserted in the middle of the cake comes out clean. Invert cake on top of an overturned mug and let cool.
• To unmold cake, loosen sides and bottom with a knife. Using your hands, gently push the cake away and downwards from the pan. remove gently and invert to a serving plate. 

Make the Pandan Icing

• Combine melted butter and icing sugar until smooth. Add the pandan extract and stir until incorporated. If icing becomes too thick, add evaporated milk one tablespoon at a time to adjust consistency. It should be spreadable but not too thick. Spread on top of the cake and serve.