The icing, incidentally, is delicious and super-chocolatey but not at all cloying. Although you will obviously feel a little unwell if you eat too much of it, the sour cream reins in the sweetness and makes it refreshing. Well, you know. Refreshing for chocolate icing.
This recipe was all done by hand, but feel free to refer to the original recipe if you have a stand mixer.
By the way, if you're thinking "what am I going to do with six egg yolks?", I recommend mixing them with hot drained pasta, mixed herbs, and plenty of cheese. Objectively delicious.
Rainbow Cake with Chocolate Sour Cream Icing
Makes a 6-layer 22x15cm rectangular cake
2 and 1/4 cups flour
1 and 3/4 cups brown sugar
1 and 1/3 tablespoons baking powder
1 teaspoon salt
170g butter (12 tablespoons)
1 cup milk
6 egg whites
1 teaspoon vanilla powder
Red, orange, yellow, green, blue, purple food colouring
1 cup sour cream
1 cup dark chocolate chips
1 cup cream
2 tablespoons butter
80g icing sugar
Make the cake batter:
In a very large bowl, mix flour, sugar, baking powder and salt. Combine and add the butter. Stir, squishing the butter with your hands if necessary, until the butter is fully incorporated and the mixture resembles wet crumbs.
Separate six eggs into yolks and whites, reserving the yolks for another use (see suggestion, above). In a different container, whisk the six egg whites with milk and vanilla. Reserve half a cup of this mixture, pouring the rest into the batter. Stir to combine. Add the remaining half-cup and stir to combine again.
The original recipe recommends dividing the batter into six equal pieces at this stage, but my kitchen equipment is not up to that task. I used a large serving spoon to remove two spoonfuls of batter into a separate bowl, and stirred in food colouring until the batter was vivid enough for me. I then lined my baking tray with parchment paper, dropped the batter into the tray, spread it out as evenly as possible, and baked for 8 minutes at 180 degrees (360 Fahrenheit).
As each layer baked, I washed up this second bowl and prepared the next layer by adding food colouring to another two spoonfuls of batter. When each layer was cooked, I lifted the parchment paper out of the baking tray and put the cake on the side to cool, replaced the parchment paper and repeated until all of the batter was gone. Two large spoonfuls each was the perfect amount because it happened to provide me with six equal-ish layers, but you may end up with a seventh if you're lucky.
Make the icing:
Put sour cream and chocolate chips into a pan over a low heat, whisking to help the chocolate melt. Add a cup of cream and two tablespoons of butter, whisking constantly. Once melted, turn the heat off under the pan and whisk in up to 80g icing sugar, until the icing has the right consistency.
Assemble the cake:
Put the red or purple, depending on which way you want the rainbow to go, on a plate or cake-decorating tray. Using a small spatula, spread icing over the top, as thickly as you'd like. Repeat with the following layers so the cake goes either: red-orange-yellow-green-blue, or purple-blue-green-yellow-orange. Turn the top layer upside down before placing it on top. Cover with icing and try to make the icing go neatly down the sides.
Refrigerate for thirty minutes before re-icing the top and sides with any leftover icing.
Refrigerate again, for about an hour, and serve. Yum.