Preheat the oven to 350°F. Add the eggs, sugar, flour, vegetable oil, melted butter, baking powder, and salt to a bowl and mix everything with a hand mixer. Pour the cake batter into a deep springform pan and bake for 90 minutes.
Remove the cake from the springform pan and let it cool off completely, then use a fork to hollow it out and create a reef.
Form clumps out of the crumbs that you removed with the fork. Place the clumps on top of the cake to refine the cake's reef look.
For the frosting, beat the butter with the powdered sugar and cocoa powder until frothy and coat the entire cake with the chocolate buttercream frosting. Take a paintbrush and dab a pattern into the cake. This works best if you refrigerate the cake from time to time. Then use the melted white chocolate to dab a pattern on top of the chocolate buttercream frosting.
Arrange the gummy fish in the reef.
Place the cake back in the springform pan and prepare the blue jello according to the instructions on the packaging.
Let the jello cool, pour half of it into the springform pan, and let it set briefly. Spread some more gummy fish all over the reef and pour the rest of the jello into the pan.
Dab the protruding 'mountains' with melted white chocolate that's been dyed with green food coloring and refrigerate the cake overnight. The next day, heat up the rim of the springform pan with a hairdryer and carefully remove the springform cake ring.