To make this crowd-pleasing dessert, measure the coconut oil in its solid state (do not melt it), and blend the cream of coconut until smooth before measuring. Martha made this recipe in episode 506 of Martha Bakes.
Cake: Preheat oven to 350 degrees. Butter and flour a 10-inch springform pan. Whisk together flour, baking powder, baking soda, and salt. In another bowl, stir cream of coconut and vanilla.
Beat butter and coconut oil on medium-high until smooth. Slowly add sugar; beat until pale and fluffy, 3 to 5 minutes. Reduce speed to medium; add eggs, 1 at a time, beating well after each and scraping down bowl as needed. With mixer on low, add flour mixture in 3 batches, alternating with 2 batches of cream-of-coconut mixture. Beat until just combined.
Topping: Whisk egg whites and salt with a mixer until thick and frothy, about 1 minute. Stir in milk and vanilla. Fold in coconut.
Spread batter in pan. Dollop topping over batter; carefully spread in an even layer. Bake on a rimmed baking sheet until a toothpick comes out with just a few moist crumbs, about 1 hour (tent with foil if browning too quickly). Let cool completely; remove from pan. Cut into wedges, drizzle with sauce, and serve with extra sauce on the side.