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 rum.
Alternate adding flour mixture and buttermilk, beginning and ending with flour (3 additions of flour and 2 of milk). Fully incorporating after each addition. Mix until just combined
Spread batter evenly into prepared pan and smooth the top with a spatula.
Bake for 30mins 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.
Place egg whites and sugar into the bowl of a stand mixer, whisk until combined.*
Place bowl over a hot water bath on the stove and whisk constantly until the mixture is no longer grainy to the touch (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 whip until smooth.**
Place all ingredients into a medium saucepan, stir to combine. Heat 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.
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 2 Tbsp of pineapple filling inside the dam and spread evenly. Top with some toasted coconut. 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 cover the outside of the cake. Decorate the top with a swirl if desired (I used a large offset spatula to do this) and press the toasted coconut into the sides.
* 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.