Preparation: Preheat the oven to 200 degrees Celsius. Place a 8-inch square tin (ungreased) into the oven to warm up while you prepare the batter.
Egg Yolks: Place the egg yolks and sugar in a large bowl. Beat with an electric mixer on high speed until the egg yolks are pale and thick to ribbon stage. Set aside.
Butter Batter: Place the butter, condensed milk and rum in a large bowl. Without washing the beaters, beat on high speed until the mixture is light and fluffy. Add in corn flour, plain flour and mixed spice and beat on low speed until fully incorporated. Beat in the beaten egg yolks on low speed until fully incorporated. Set aside. Wash and dry the beaters.
Egg Whites: Place the egg whites in a large, clean, dry and oil-free bowl. Beat on low speed until foamy. Add in the salt and beat with an electric mixer on medium speed until soft peaks formed. While the mixer is still running, add in the sugar gradually. Once all the sugar has been added, beat on high speed until the egg whites are glossy and stiff peaks formed. Fold ⅓ of the egg whites into the butter batter. Fold it in to lighten the mixture (don't have to be too gentle at this stage). Fold in the rest of the egg whites gently until fully incorporated.
Baking First Layer: Weigh 100 grams of batter. Remove the hot tin from the oven. Brush the bottom generously with the melted butter, trying your best not to get the butter onto the sides, otherwise your first layer may shrink away from the sides of the tin.
Pour the batter into the tin and spread it evenly, pushing the batter to the corners of the tin. The batter will "melt" but it's okay. Bang the tin twice and bake the cake for 5 to 7 minutes on the center rack until the top is golden brown in colour.
Baking Subsequent Layers: Weigh 100 grams of batter. Remove the tin from the oven. Change the oven onto the grill setting (only the top heating element is on) and increase the temperature to 200 to 240 degrees Celsius. Poke the first cake layer all over with a toothpick. Press the cake layer down lightly with a cake presser to flatten the cake. Pour in the batter and spread it evenly, pushing the batter to the corners of the tin. The batter will "melt" as the bottom layer is warm but it's okay. Bang the tin twice and grill the cake for 1 to 3 minutes until the top is golden brown in colour (I prefer a darker golden brown colour). Repeat the process.
Update 7/2/2014: To be more specific, I made a total of 15 layers with the batter. I grilled the 2nd to 5th layers at 240 degrees Celsius for about 2 minutes 30 seconds (per layer). The 6th to 10th layers are grilled at 220 degrees Celsius also for about 2 minutes 30 seconds (per layer). The 11th to 14th layers are grilled at 200 to 210 degrees Celsius, for about 1 minute 45 seconds to 2 minute 15 seconds. I have a small oven, so the top layers are very close to the heating element, so I had to watch the layers very closely to prevent the layers from burning. Also, it helps to know the "hot spots" in your oven so you know when to rotate your tin to get an even browning for each layer.
Baking Last Layer: Follow the same steps as the "Subsequent Layers". Change the oven back to normal settings (top and bottom heating element is on) and reduce the heat to 180 degrees Celsius. Bake for 8 to 10 minutes until the top is golden brown in colour. Remove the cake from the oven and invert it out on a wire rack. If the cake did not fall out on its own after 15 minutes, run a sharp knife around the edges of the cake and invert it out. Cool the cake completely on the wire rack.
Storing and Serving: Once the cake has cooled completely, store the cake in a container. Let the cake rest at room temperature for one day for the flavours to develop. Slice the cake and serve. The cake can be kept at room temperature in a container for 1 week or in the refrigerator for up to 3 weeks.