Preheat your oven to 350 degrees F and grease a 26cm (10 inch) round spring form pan with butter. In a small bowl, sift together the spelt flour, cornstarch, cocoa powder and baking powder. Set aside.
Separate the eggs, adding the egg yolks to a large bowl or stand mixer bowl and the egg whites to another large bowl. Using the whisk attachment, whisk the egg yolks with the water until frothy then add approximately 2/3 of the sugar and the vanilla extract and whisk again until thick and creamy. Set aside.
In a stand mixer with the whisk attachment or a large bowl with a hand mixer beat the egg whites until very stiff peaks. Add the remaining sugar and whisk again to combine. Gently empty the egg white mixture onto the egg yolk mixture and then add the sifted dry ingredients on top. Using a large spatula, gently fold together all the ingredients until evenly combined.
Pour mixture into the greased spring form pan and bake for 35 minutes or until a toothpick inserted into the centre comes out clean. Remove from the oven and set aside to cool. Cover the cake and chill in the fridge for one hour before filling (this will make the cake easier to slice into layers).
In a medium bowl sift together the icing sugar and cocoa powder. Set aside. In the bowl of a stand mixer with the paddle attachment or a large bowl with a hand mixer beat the butter until creamy. Add the sifted ingredients along with the whipping cream, vanilla extract, sea salt and rum (if using) and beat on high for one full minute. Set aside 12 tsp balls of this mixture for decoration on a plate and chill in the fridge.
Remove the cake from the spring form pan and using a large, serrated bread knife cut the cake into three equal layers. Place the bottom layer on a cake plate or platter and spread half the chocolate buttercream on top. Repeat with the second layer. Top with the last layer of cake. Set aside.
Make the icing by whisking the icing sugar, cocoa powder and hot water in the bowl of a stand mixer with the whisk attachment or a large bowl with a hand mixer. Spread the icing evenly over the top and sides of the cake.
Sprinkle the chocolate sprinkles around the base of the cake then lift and gently press them onto the sides of the cake with a spatula. Sprinkle more around the top of the cake in a 1 inch border.
Pour additional chocolate sprinkles in a small bowl. Roll the 12 tsp of buttercream you set aside to chill into small balls, roll each in the chocolate sprinkles and place on the cake. Chill the entire cake in the fridge to set the icing before serving.