Preheat oven to 350F. Grease and flour three 6" cake rounds, line with parchment.
In a medium bowl, whisk flour, baking powder, and salt and set aside.
Beat butter until smooth. Add sugar and beat on med-high until pale and fluffy (2-3mins).
Reduce speed and add eggs one at a time, fully incorporating after each addition. Add vanilla and rum emulsion.
Alternate adding flour mixture and coconut milk, beginning and ending with flour (3 additions of flour and 2 of milk). Fully incorporating after each addition. Mix until just combined. Fold in shredded coconut.
Spread batter evenly into prepared pan and smooth the top with a spatula.
Bake for 30-35 mins or until a toothpick inserted into the center comes out mostly clean.
Place cakes on wire rack to cool for 10mins then turn out onto wire rack to cool completely.
Rum Simple Syrup:
Place sugar and water into a pot over med-high heat. Stir to dissolve sugar and cook until mixture boils. Simmer for 1 minute. Remove from heat and stir in rum (or rum extract/emulsion). Cool before use.
Coconut Rum Buttercream:
Place egg whites and sugar into the bowl of a stand mixer, whisk until combined.*
Place bowl over a pot with 1-2" of simmering water and stir constantly with a whisk until the mixture is hot and no longer grainy to the touch or reads 160F on a candy thermometer (approx. 3mins).
Place bowl on your stand mixer and whisk on med-high until the meringue is stiff and cooled (the bowl is no longer warm to the touch (approx. 5-10mins)).
Switch to paddle attachment. Slowly add cubed butter and mix until smooth. Add rum and coconut milk powder one Tbsp at a time and whip until smooth.**
Place all ingredients into a medium saucepan, stir to combine. Bring to a simmer and cook over medium-high until thickened (1-2 mins). Cool completely before using on cake.***
Preheat oven to 350F and line a large sheet pan with parchment. Spread flaked coconut out evenly and bake for approx. 5 mins or until golden brown. Toss lightly part way through baking. Cool completely before using on cake.
Trim the top crust off of each layer of cake. Dip a pastry brush into the rum simple syrup and brush each layer generously (about 2-3 Tbsp per cake layer).****
Place one layer of cake onto a serving plate or cake stand. Top with a thin layer of buttercream and pipe a dam around the outside using a large round tip to hold the filling. Place 1/3 cup of pineapple filling inside the dam and spread evenly. Repeat with next layer.
Top with final layer of cake and crumb coat the entire cake (thin coat of frosting all over the cake). Chill for 30mins.
Use the remaining frosting to frost the cake and do a rustic swirl on the sides and top with a large offset spatula if desired. Press large coconut flakes along the bottom of the cake.
Space 6 pineapple wedges evenly around the top of the cake. Pipe rosette dollops in between them using a 1M tip. Top each dollop with a maraschino cherry. Fill in the center with toasted shredded coconut if desired.
* Ensure there is no trace of egg yolks in your whites and that your mixer bowl and whisk is completely grease-free or your meringue won't stiffen.
** The buttercream may look like it's curdled at some point. Keep mixing until it is completely smooth. See this post for a detailed tutorial.
*** You will have some pineapple filling left over. If you can find an 8 oz can of crushed pineapple you can halve the recipe and use that instead.
**** You will have some rum simple syrup left over. You can store this in the fridge and use this in cocktails, coffee, iced drinks, etc.