Preheat the oven to 300°F.
In a medium bowl, whisk together flour, 1 cup of sugar, and baking powder. Set aside.
1 1/2 cups (212g) all-purpose flour, 1 1/2 cups (300g) granulated sugar, 1 teaspoon baking powder
In a separate bowl, beat egg whites until they are white and foamy. Gradually add the remaining 1/2 cup of sugar and beat until stiff peaks form. Set aside.
6 large eggs
In a large bowl, whisk egg yolks with lemon juice and pineapple juice.
1 Tablespoon fresh lemon juice, 1/2 cup pineapple juice
Add 1/4 of the flour mixture to the egg yolks, stirring gently just until combined. Add in another 1/4 of the flour mixture.
Gently fold in stiff egg whites using a rubber spatula, using as few strokes as possible.
Add remaining flour mixture, folding it in until everything is just combined using as few strokes as possible so as not to deflate egg whites. Transfer the batter into a clean, ungreased tube pan. Run a knife through batter to remove air bubbles.
Bake for 60 minutes until done. Cake can be tested with a toothpick to see if it comes out clean.
Immediately invert the pan over a cooling rack or cake plate, but do not attempt to remove the tube pan until the cake has cooled completely. While the cake cools, make pineapple frosting.
For frosting, beat softened butter and lemon zest in a bowl using a mixer until light and creamy.
3 Tablespoons salted butter, 1 Tablespoon lemon zest
Add powdered sugar gradually, mixing on low speed until combined.
3 cups (360g) powdered sugar
Mix in lemon juice, pineapple juice, and salt, beating for 3 minutes until frosting is light and fluffy.
1 Tablespoon lemon juice, 3 Tablespoons pineapple juice, Pinch of salt
Spread over cooled cake, then slice and serve.