Prehat oven 350ºF. Butter and flour a 9X13" cake pan.
Place the flour, baking powder and salt in a large bowl and whisk together.
Place the egg yolks in a mixing bowl with ¾cup sugar and beat on high speed until pale in color. Stir in the coconut milk and vanilla. Pour over yolk mixture over the flour mixture and stir gently until combined.
Clean the mixing bowl and add the egg whites. Beat on high until soft peaks form. While running, add the remaining ¼cup sugar. Beat until egg whites are stiff but not dry. Pl;ace egg whites in the batter bowl. Fold them gently into the batter mixture. Spread evenly into prepared cake pan. Bake for 25 to 25 minutes, or until a toothpick comes out clean when inserted in the center. Turn out onto patter and allow cake to cool for at least 10 minutes.
While cake is baking blend the evaporated milk, condensed milk and coconut milk well in a small pitcher or large jar. Once cooled for 10 minutes, poke the cake all over with a fork. Brush the Grand Marnier on top, if using. Slowly pour all but 1 cup of the milk mixture all over cake and allow to soak for 30 minutes. (You will have one cup extra, that can be discarded.)
Meanwhile whip the heavy cream until soft peaks form. Add the sugar a whip about 30 seconds more. Spread evenly over cake and top with toasted coconut. Keeps will in fridge, covered for up to a week.