Prepare Pandan: Wash and blend the pandan leaves with 2 Tbsp water using a food processor or blender. Strain and discard the solids.
Pandan Chiffon Cake: Using an electric whisk, beat egg whites and cream of tartar until it is foamy and increasing in volume. Then slowly add in 1/4 cup sugar and continue whisking for a few minutes until stiff, glossy peaks form.
In another bowl, combine the dry ingredients (flour, baking powder, salt).
Preheat oven to 325ºF.
In a third bowl, whisk the wet ingredients (egg yolks, 1/2 c sugar, oil, vanilla) until pale yellow. Mix in the pandan juice.
Alternatively add in 1/3 of the flour mixture and 1/3 of milk to the wet ingredients, and continue adding the batches and whisking until well incorporated.
Using a spatula, gently fold in the egg whites. This is what makes the cake light, so if in doubt, under mix. It's okay to have streaks of white and green.
Spoon cake batter into a cupcake pan, ungreased. Bake for 20 minutes. Then increase oven temperature to 350ºF and continue baking for about another 5 min, or until cupcakes turn golden brown and toothpick comes out clean. Cool cupcakes on a cooling rack. Use a small spatula to help remove the cupcakes from the pan. The cakes will be green inside even if the outside is golden.
Coconut Frosting: While the cake bakes, make the frosting. Combine the egg whites, sugar and cream of tartar on the top half of a double boiler on low heat.
Using an electric whisk, beat the egg mixture until it begins to thicken. Then add in vanilla extract and continue whisking on med-high till stiff peaks form. Remove from heat and stir in coconut. Let it cool to about room temperature before use.
Once the cakes and frosting has cooled to room temperature, frost each cupcake and enjoy!
Notes
Pandan can be found in many Asian grocery stores, and you can also use frozen pandan, thaw before use