Cook the pineapple, sugar, water, and optional cinnamon over medium-low heat for 15–25 minutes, stirring occasionally, until thick and jam-like. Add the cornstarch slurry during the last few minutes if needed, remove the cinnamon, and cool completely.

Heat the oven to 350°F/175°C. Grease and flour two 9-inch round pans and line the bottoms with parchment.

Whisk together the flour, baking powder, and salt. Cream the butter and sugar for 4–6 minutes, then add the eggs one at a time, followed by vanilla and citrus zest.

Add the dry ingredients in three additions, alternating with the milk in two additions. Mix only until combined, divide between the pans, and bake for 35–45 minutes.

Cool the cakes in their pans for 10–15 minutes, then transfer them to racks to cool completely. Level if needed, spread the pineapple filling between the layers, and prepare the meringue.

Boil the sugar and water to 240°F/116°C while whipping the egg whites until foamy. Gradually pour in the syrup while beating, continue until glossy and stiff, add vanilla, and frost the cake immediately.