I covered and filled the cake with a simple cream cheese icing, then made my own blue sugar crystals to finish.
The cake went down really well, and stayed moist for days afterwards! The cake was finished off today but had lasted nearly a week in an airtight container.
Also, there's still loads of time if anyone would like to donate to the amazing Christie's charity - they have one of the largest cancer centres in Europe and have saved so many lives. I'm running a marathon in April, and would love to raise as much money as possible for them - my justgiving page is here.
Makes one 23cm cake
For the cake:
- 110g (1/2 cup) softened butter or margarine
- 250g (1 & 1/4 cups) granulated or caster sugar
- 2 eggs
- 1 tsp vanilla extract
- 225ml (1 cup) natural yoghurt or buttermilk
- 2 tsp blue food colouring (I used a concentrated gel)
- 280g (2 cups) plain flour
- 2 tbsp cocoa powder
- 1 tsp salt
- 1 tsp bicarbonate of soda
- 1 tsp white vinegar
For the cream cheese frosting:
- 110g (1/2 cup) softened butter
- 225g (1 cup) cream cheese
- 480g (4 cups) icing sugar, sifted
- 2 tsp vanilla extract
For the blue crystals:
- 100g (1/2 cup) caster sugar
- 35ml (1/4 cup) water
- toothpick dipped in concentrated blue food colouring
1. Grease the base and sides of two 23cm springform circular cake tins. Line the bases with baking parchment.
2. Preheat your oven to 180c (160c fan)/355f/ gas mark 4.
3. Cream the butter or margarine against the sides of a bowl until it is really soft and spreadable. Add the sugar and cream together until light and fluffy.
4. Add the eggs, vanilla, food colouring and yoghurt/buttermilk and one tablespoon of the plain flour, and whisk until smooth.
5. Sift in the remaining plain flour, cocoa powder and salt. Use a large spoon to fold the flour/cocoa/salt into the cake mixture, until no flour speckles can be seen.
6. In a small bowl whisk together the bicarbonate of soda and vinegar. Immediately pour into the cake batter and fold in.
7. Evenly divide the batter between the two cake tins. Bake for 25-30 minutes, or until the top is springy and a skewer inserted into the centre of the cake comes out clean. Leave to cool in the tins.
8. To make the frosting, cream the butter against the side of the bowl until it is very soft and spreadable.
9. Add the cream cheese and beat with the butter until smooth. Pour in the vanilla extract and half of the icing sugar, and beat until all of the sugar has been combined. Add the other half of the icing sugar and beat until smooth.
IF you have lumps of butter in your icing, don't stress! I've had this problem before, and have found an easy fix. Use a hand blender to blitz the frosting - it will go lovely and smooth, and didn't lose firmness :)
Place in the fridge until ready to use.
10. Grease and line a baking tray with greaseproof paper.
11. Make the blue sugar crystals by pouring the sugar and water into a medium saucepan. Place on a low heat, and heat until all of the sugar has dissolved.
12. Turn the heat up to medium, and heat (WITHOUT STIRRING). It will bubble up pretty ferociously, and after about 5 minutes, the bubbles will settle down to a nice rumble - when it gets to this stage keep a close eye on it. At that point, swirl the pan every minute or so.
12. Once the colour turns a golden colour (on a sugar thermometer it will read 149 - 154c/300-310f), dip the food coloured toothpick into the sugar. Swirl until the sugar has been evenly coloured, then pour onto the lined baking tray. Leave to set for around 10 minutes, until cold. If you want to make shards rather than small crystals, spread the blue caramel around the tray so that it is around 2mm thick.
13. Break into shards. Leave them like this if you like, or crush the sugar using a rolling pin or high powered food processor.
14. To assemble the cake, remove both cakes from the tins, and use a serrated knife to level the top of the cakes off (effectively removing the dome formed on top of the cake).
15. Place a small amount of icing on the middle of the cake board/serving tray, and top with the first cake layer. This acts like glue so that the cake won't slip!
16. Place around a third of the icing on top of the first cake layer, and smooth over the top. Top with the other cake layer.
17. Spread a small amount of the icing around the sides and top of the cake using a palette knife. This doesn't need to look neat - it acts as a crumb coat, meaning that when you put the final layer of icing on, you won't see any blue crumbs in the final finish.
Place the cake in the fridge for 10-15 minutes for the icing to become less sticky.
18. Place a large spoonful of icing on the top of the cake, and use a palette knife to evenly smooth over the cake top. Then add a big spoonful of icing on the side of the cake, and smooth this around the sides. Once the sides of the cake have been covered, dip the palette knife in hot water, dry it, then smooth around the cake. This will give the neatest finish.
19. Place the sugar crystals around the base of the cake, or where wished. This cake was made for my sister Gwyneth, so I did a big G on the middle of the cake :)
20. Place in the fridge to set for around half an hour. I had mine in a cake box overnight (so easily over 12 hours) and the icing didn't melt.