Astrobaking: 3D Mercury Cake

Pay tribute to the rocky planet Mercury with Katharine Kilgour's spherical cake, complete with chocolate truffle core! Katharine Kilgour likes to combine her love of astronomy with her love of cooking, making tasty treats to feed herself, her friends and her family Astro-boy (aged 12), Astro-girl (aged 16) and Astro-dad.

M3 cake and slice

Bake your own Jovian system following Katherine’s recipe below. All images: Katherine Kilgour


Mercury is a planet that is hard to see, only visible with a telescope before dawn or after sunset.

Not to mention the fact that the eastern and western horizons are below the treeline from our garden, requiring a trip with the telescope and very precise timing to catch the planet in the sky while the sun is not.

Mercury is also easily mixed up with Earth’s Moon, particularly in high speed games of Astro-snap!

However, although it is hard to see and not the most colourful object it is still an interesting world. Its proximity to the Sun and slow rotation give it some of the most extreme temperatures in the Solar System.

The planet is thought to be like Earth with a crust, mantle and core, which rather lends itself to a being a truffle-filled cake!

Astro-girl and Astro-boy know that the Moon is not made of cheese; they have even held Moon rock in their hands to prove it. Perhaps I can convince them that planets can be made of cake!

Cross section of Mercury

Given the proliferation of spherical objects in the Universe, a set of hemisphere cake pans is a sensible investment for the astro-baker.

They are widely available online or in cook shops. I even found a set in one of the discount supermarkets.

Just make sure they have a ring to sit on to prevent them rolling round in the oven, or use a couple of metal cutters.

My tins have a 15cm diameter.

If yours are different then you will need to adjust quantities of mixture.


For the mantle:
250g soft margarine
250g caster sugar
5 eggs
300g self raising flour
1 teaspoon vanilla essence


For the core
50g dark chocolate
50ml double cream 


For the crust
30g butter
200g marshmallows (white are best but if there are a few pink ones in the mix it won’t matter)
120g crisped rice cereal
Black food colouring


For the mantle sponge:



M3 step 1b

Place your hemisphere pans on their rings on a baking tray.

Spray them with ‘quick release cake spray’.

It really is worth using this as the cakes will slip out of the tins easily and the shape makes it impossible to ease them out with a knife or other implement!



M3 step 2

Beat together the margarine and sugar.



M3 step 3

Add the vanilla and the eggs, one at a time, mixing thoroughly with each addition, then fold in the flour.



M3 step 4

Divide the mixture between the two hemisphere pans.



M3 step 5

Bake at 180°C until risen and golden, approximately 45 minutes.

When cooked, a skewer inserted into the middle will come out clean.



M3 step 6

Allow them to cool then use a knife to slice off excess cake mixture using the cake tin rim as a guide.



M3 step 7

Turn the cakes out onto a wire rack and leave until completely cold (or they will melt the core!)

For the chocolate truffle core



M3 step 8

Melt the chocolate either in a microwave or over a pan of hot water.



M3 step 9

When completely melted, whisk in the cream.

Whisk until cool and then refrigerate until the mixture solidifies.



M3 step 9


Melt the mallows

Place the butter and marshmallows in a large saucepan and melt over a gentle heat. Keep stirring until the mallows have melted and the mixture is smooth.

M3 step 11a

Do not allow it to boil.

Add enough black food colour to get a good colour for the planet Mercury.

M3 step 11b



M3 step 12

Stir in the crisped rice cereal and ensure it is well coated in the marshmallow mixture.


Working the hemispheres

Keep the mixture warm while you work to coat the two cake hemispheres.

This mixture is very sticky!

A metal flat knife and your hands are the best implements to use.

Dip the knife in hot water and use it to help smooth the mixture.


Sandwich and gaps

M3 step 14

Once the two cakes are coated you can sandwich them together on your serving plate.

Use the remaining crispy mixture to fill in any gaps.



M3 step 15

Allow the cake to stand so that the mixture firms up, before slicing to reveal the planetary layers.