Preheat your oven to 350°F (175°C). Grease and flour a 9x13-inch baking pan.

In a medium bowl, whisk together the all-purpose flour, baking soda, and salt until well combined.

In a large bowl, cream together the granulated sugar and softened unsalted butter using an electric mixer on medium speed until light and fluffy.

Add the eggs one at a time, beating well after each addition, then mix in the vanilla extract.

Gradually add the flour mixture to the wet ingredients, alternating with the buttermilk, beginning and ending with the flour mixture.

Fold in the crushed pineapple, shredded coconut, and chopped walnuts using a spatula until evenly combined.

Pour the batter into the prepared baking pan and spread it out evenly.

Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted into the center comes out clean.

Remove from the oven and allow to cool completely in the pan on a wire rack.

While the cake is cooling, prepare the frosting by beating together the softened cream cheese and unsalted butter in a medium bowl until smooth and creamy.

Mix in the vanilla extract, then gradually add the powdered sugar, 1/2 cup at a time, beating until smooth and fluffy.

Stir in the pineapple juice until the frosting reaches a smooth consistency.

Once the cake has cooled completely, spread the frosting evenly over the top.

Slice and serve the cake, enjoying the flavors of pineapple, coconut, and walnuts.