For the puree:
1 cup sugar
1 cup water
1 pineapple, peeled, cored, and chopped
For the cake:
1 cup unsalted butter, softened, plus additional for pans
3 cups cake flour, plus additional for pans
1 tablespoon baking powder
1 teaspoon salt
1 cup cream of coconut
¼ cup whole milk
1 teaspoon vanilla extract
½ teaspoon coconut extract
1¼ cups sugar
2 large eggs
3 large egg whites
For the buttercream:
4 egg whites
1½ cups sugar
Pinch of salt
3 sticks (1½ cups) unsalted butter, cut into tablespoons, at room temperature
1 to 1½ cups pineapple puree
1-2 tablespoons coconut rum (optional)
To make the puree, combine the sugar and water in a medium saucepan. Cook over medium heat, stirring often, until the mixture reaches a boil. Add pineapple and return to a boil without stirring. Remove from heat, cover and allow to sit overnight. The next day, drain the syrup, reserving for the cupcakes, and puree the pineapple until smooth.
To make the cupcakes, heat oven to 325° and line two cupcake pans with liners. In a medium bowl, whisk together the flour, baking powder and salt. In a 2-cup measuring cup, whisk together the cream of coconut, milk and extracts. Set aside.
Place the butter and sugar in the bowl of an electric mixer. Beat on high speed until light and fluffy, about 3 minutes. Add the whole eggs and egg whites, one at a time, beating well after each addition. Reduce speed to low and mix in the flour mixture and cream of coconut mixture alternately in batches, beginning and ending with the flour. Beat until just combined.
Divide batter evenly among the prepared liners and bake for 22-25 minutes, or until a toothpick inserted in the center comes out clean. Cool in pan for about 10 minutes, then transfer to a wire rack to cool completely before frosting.
To make the frosting, combine the egg whites, sugar and salt in the bowl of a stand mixer set over a small saucepan of simmering water. Whisk constantly by hand until the sugar has dissolved. Test by rubbing the mixture between your fingers; you shouldn’t feel any grittiness.
Attach the bowl to the stand mixer and fit with the whisk attachment. Begin mixing on low speed and gradually increase to high and whip until stiff peaks form. Continue mixing until the mixture is completely cool (test by feeling the bottom of the bowl) and light, fluffy and glossy. This should take about 10 minutes but can take longer.
Replace the whisk attachment with the paddle attachment. Set the mixer to medium-low and add the butter, 2 tablespoons at a time, mixing well between each addition. Once all the butter has been incorporated, increase to medium speed and beat until fluffy. Gradually add the pineapple puree and rum, and mix to combine.
Brush the tops of the cakes with the pineapple syrup, then frost as desired.