Special equipment: two 16-centimeter-by-8 centimeter (6 1/4-inch-by-3-inch) hemisphere cake pans; 2 large round cutters; 3 pastry bags fitted with 2 round tips and 1 multi-opening #233 tip
For the cake: Preheat the oven to 320 degrees F/160 degrees C. Grease two 16-centimeter-by-8 centimeter (6 1/4-inch-by-3-inch) hemisphere cake pans.
Cream the butter and superfine sugar together with an electric mixer until pale and fluffy. With the machine running, slowly add the eggs. Add the lemon zest. Add the flour, baking powder and salt. Add the lemon juice. Mix well.
Evenly divide the cake batter between 2 bowls. Color one with red and one with yellow food coloring. Scoop the batter in alternating colors into the prepared cake pans. Swirl the batter to create a marble effect. Sit the cakes on top of 2 large round pastry cutters on top of a baking sheet. This will keep the pans stable as they bake. Bake until a toothpick comes out clean, about 50 minutes. Cover with foil if the cakes begin to brown too much.
Let the cakes cool in the pans for 5 minutes, and then turn them out onto a cooling rack to cool completely.
For the buttercream icing: Beat the butter with an electric mixer. On low speed, beat in the confectioners' sugar. Beat on medium-high until light and fluffy. Beat in the milk. Evenly divide the icing among three bowls. Tint one bowl with blue food coloring and another with green food coloring. Leave one bowl white. Fill a pastry bag fitted with a round tip with the blue icing. Fill a second pastry bag fitted with a multi-opening #233 tip with the green icing. Fill a third pastry bag fitted with a round tip with the white icing.
Carve a small crater out of the middle of each cake. Spread some blue icing on a cake board or serving plate. Using a serrated knife, cut a small slice off the round side of one cake and place the cake round-side down on the cake board. Fill the crater with sprinkles. Spread more blue icing along the edges of the cakes and stick the two together to create a globe. Spread blue icing on the outside of the cake. Pipe on green icing for the land. Create the poles with white icing. Pipe a little white icing to create ripples and waves in the ocean. Add edible sliver leaf for some sparkle, if using.
Cut the cake to find the surprise inside!