Preheat oven to 350°F. Line 2 quarter sheet pans (9 inch x 13 inch) with parchment paper, and grease with non-stick cooking spray.

Place 2 flower nails upside down in the pans, spaced a few inches apart from one another. Spray generously with non-stick cooking spray.

Mix together all dry ingredients (flour, baking powder, sugar, and salt) in a stand mixer with a paddle until fully combined.

Mix chunks of room-temperature butter slowly into the dry mix, on a low speed. Continue to mix until no large chunks of butter remain, and the mixture becomes crumbly.

Pour in egg whites, and mix on low until just incorporated. Slowly mix in the pineapple juice, then add the buttermilk, and mix on a low speed.

Add in vanilla and oil, and mix at a low speed until fully incorporated. Scrape down the sides of the bowl with a spatula, then beat on medium speed for about 30 seconds. 

Divide batter evenly between the prepared cake pans.

I like to use a digital kitchen scale to weigh my pans, and ensure they all have the same amount of batter. This guarantees the layers bake to be the same height.

Bake for 34-36 minutes, or until a toothpick comes out clean. Allow the pans to cool for 10 minutes, then run an offset spatula around perimeter of the pan to separate the cake from the pan.

Place cake layers into the freezer for 30 minutes, to accelerate the cooling process.Once the layers are fully cooled, carefully flip the pans and remove the layers from the pans.

Use a serrated knife to level the tops of the layers, the cut each sheet cake in to three rectangular pieces (7 in x 4.33 in pieces).

Place the pineapple juice and granulated sugar in a sauce pan, and heat at a medium high heat.

Stir the mixture occasionally until it begin to boil.

Remove from heat, and pour into a separate bowl to cool.

Set aside.

While the cake layers bake and cool, make the vanilla buttercream frosting.

Beat the butter on a medium speed for 30 seconds with a paddle attachment, until smooth. Slowly add in the powdered sugar, 1 cup at a time. Alternate with small splashes of cream.

Once fully mixed, add in the vanilla and salt, and beat on low until the ingredients are fully incorporated, and the desired consistency is reached. 

If the frosting is too thick, add in additional cream (1 teaspoon at a time). If the frosting is too thin, add in more powdered sugar (quarter of a cup at a time).

Place 1 /2 cup of uncolored frosting into a piping bag, and cut a 1 inch opening at the end of the bag. Set aside.

Place 1 /2 cup of frosting into a separate bowl, and mix in 1 tbsp of cocoa powder. Place this chocolate frosting into another piping bag, and cut a 1 inch opening in the bag. Set aside.

Color the remaining frosting yellow with gel food coloring.

Brush each cake layer with the pineapple simple syrup using a kitchen brush.

Stack and frost cake layers on a large, 12 inch greaseproof cake board, using a dab of frosting to help stick the first cake layer to the board.

Add an even layer of yellow buttercream between each cake layer, and a sprinkle of rainbow jimmies.

Insert either a central wooden dowel, or two bubble tea straws into the cake as supports.

Apply a thin coat of frosting around the the cake, to fully cover the cake layers. Smooth using a bench scraper, then chill the cake in the fridge (10 minutes) or freezer (5 minutes) until the frosting is firm to the touch.

Add a second, thicker layer of frosting to the top 3/4 of the cake, and smooth using a bench scraper.

Pipe a white line underneath the yellow, then a brown line in the remaining space.

Smooth using a bench scraper and an acetate sheet.

Use fondant to create SpongeBobs facial features, tie, belt, and shoes.

Carefully press the fondant onto the buttercream.

Pipe on SpongeBob's legs and arms.

Use a small spoon dipped in warm water (and dried) to make SpongeBob's holes.