Grease the sides and base of an 8-inch square cake pan and line the base with greaseproof paper. Set your oven rack right in the centre of the oven. Pre-heat oven to 200 deg C (390 deg F).
Sift the flour and mixed spice together into a mixing bowl. Set aside. Reserve some prunes for topping the cake.
Using an electric mixer fitted with a paddle attachment, beat butter with condensed milk on medium speed (speed 3 on my Kitchen Aid mixer), until light and fluffy, about 10 minutes. Set aside.
Switch to the whisk attachment. In a clean mixer bowl, whip whole eggs, egg yolks, sugar and sponge gel (Ovalette) on high speed (speed 5 on my Kitchen Aid mixer) until mixture turns pale, becomes thick and fluffy, about 7 to 8 minutes.
Now, switch back to a dry and clean paddle attachment. Reduce mixer speed to low (speed 1 to 2). Add the flour mixture to the egg mixture and beat until just combined. Maintaining low speed, add butter in 3 batches, beating well after each addition.
Heat the greased cake pan in the oven for 2 minutes. Set the oven to grill (or broil) mode.
Scoop batter using a soup ladle, and spread evenly into the bottom of the pan. Using the back of a metal spoon, level the surface (add more batter if necessary - I needed about one-and-a-half scoops of a soup ladle per layer). Grill for about 5 to 6 minutes, or until layer is cooked and nicely browned.
Remove pan from the oven. Prick a few holes lightly with a fork, and press lightly on the cooked layer with a lapis press (optional) to remove air pockets. Add the next layer of batter (now that you've determined how much you need), spread and level the surface, and grill. Repeat until all the batter is used up.
Scatter some prunes on the uncooked batter every 4th layer (i.e. 4th, 8th, 12th layer, etc) and grill as before.
As the layers build up and get closer to the top heating element, grilling time will be less, so keep a careful watch so as to avoid burning any layer. For the last 2 to 3 layers, move oven rack one level down.
On the final layer, scatter the top with the reserved prunes. Set the oven to 180 deg C (390 deg F) and switch to upper and lower heat. Bake the cake for another 10 minutes before removing from the oven.
When done, allow cake to rest in the pan for 10 minutes. Remove from pan and allow cake to cool completely. Trim all around the sides to reveal the layered (lapis) pattern. Slice thinly and serve.