Preheat oven to 350°F/177°C. Grease and line a 9x13x1-inch rectangle cake pan. Set aside.
In a bowl, mix together cake mix, melted butter, egg and water until smooth. Pour into prepared cake pan.
Bake in the preheated oven for 15-20 minutes or until set.
Set aside cake and then refrigerate to cool further. Cut the cake into rounds using a 2-inch round cutter.
Prepare buttercream by boiling the sugar and water until thread like. Using the whisk attachment for a stand mixer, whip the yolks until thick. Gradually pour in cooked syrup and continue whipping until thick. Continue whipping to cool the mixture slightly. Set aside. In another bowl, beat the butter until soft and creamy. Add in the cool yolk-sugar mixture and then beat until it has a spreading consistency.
To assemble, take a layer of the round cake, spread with buttercream, and sandwich with another cake layer. Cover the entire cake with more buttercream then roll in desired coating.
Refrigerate to set. Serve.