My sister's 30th birthday party was 1980's themed (since she was born in 1980). We came up with the homemade Rubik's cube cake as ideal for the 80's. This is one for chocoholics - the whole cake is chocolate based - cakes, ganache and the coloured squares.
I made 3 types of chocolate mud cake - 1 for each layer of the cube - dark, white and caramel. I baked them in a 20cm square cake tin with 5cm height. The height and width of the pan meant that after leveling and trimming the cakes to perfect squares that I could make the coloured squares for the cube 4.5cm squares with a gap of 2mm between each row.
I don't like using buttercream to ice cakes - I don't like the taste. So I use chocolate ganache where I can. For the icing on this cake I made dark chocolate ganache by heating thickened cream til just boiling and then poured it over 45% cocoa solids couveture chocolate and mixing til smooth. I added black liquid food colouring to make the ganache even darker - so it looked like the black on a Rubiks cube. You have to let it thicken a little - in the fridge if the weather is warm. If the ganache sets too quickly you can gently heat in the microwave on low til desired spreadability achieved.
I froze the cakes individually ahead of time. This makes them so much easier to ice - the crumbs don't come off and get mixed in the icing. I put 2 cakes on top of each other exactly and the top layer was turned 45 degrees. I stuck the bottom cake to the board with a bit of the ganache and then put a layer of ganache in between each cake layer. Having frozen cakes makes the ganache set quickly.
I do a crumb coat first - this a thin layer of icing/ganache that makes any crumbs stick to it - especially if you have had to trim the edges of a cake. Then I put the cake in the freezer to set it for 10 minutes. Next step is to do the thicker top coat of ganache - one side at a time - so you can stick your coloured squares to the cake while the ganache is still soft. You can place a little extra ganache on the back of each square if desired.
The coloured squares are made from white chocolate that are tinted with oil based food colour powders - NOT liquids! Liquid will seize the chocolate. I melted the white chocolate melts (about 100grams per colour) and then kept whisking in powder colour til the desired colour was achieved.
I drew a 9 square grid on a piece of paper with the lines extended beyond the grid for cutting lines. Then placed a piece of baking paper on top of the grid. I stuck the papers to a marble chopping board and then spread 1 chocolate colour over the baking paper - about 2mm thick. When the chocolate was just set - you don't want it too hard or it becomes brittle - I used a thin bladed cleaver (miracle blade) to cut squares by following extended grid lines. Repeat with other colours. You will have more squares than you need because of the side that is stuck to the board. But to make an authentic mixed up cube you will need to mix up all the colours and you inevitably break some or find some have not set smoothly.
Lay out 6 sides of the mixed up cube on baking paper, with the smooth side facing up, making sure the centre square of each side is different. As I iced one side of the cube I transferred the coloured squares to the iced side of the cube, leaving a gap around each square for the black icing to show through. If you find that your fingers have left prints on the squares you can gently blowdry on low, the surface of the cake to gloss the squares and icing. When they re-set they will be smooth. You can do this to the icing before putting the squares on if the icing has set too quickly.
It seems like a lot of work to create this homemade Rubik's cube cake, but if you prepare all the parts (cakes frozen in 2 layers of cling wrap and chocolate squares on sheets of baking paper, stored in airtight containers) in the days ahead of assembling, it's not as big a job as it seems. The end result will get you lots of praise!