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  • Writer's pictureAndrew Smyth

Chocolate Geode Cake

What a weekend! I had a whale of a time at Maker Faire UK in Newcastle, my first time at the event and in the city and I will definitely be back to both! It was inspiring, filled me with enthusiasm and was just amazing fun. As part of the event, I gave 6 demonstrations showing the science behind geodes and also how to make a geode cake at home! Layers of rich chocolate madeira cake sandwiched with sharp morello cherry jam and smothered in vanilla buttercream. As for the geode part, I opted for a cheat quick and easy method with great results that is ready in a matter of hours! You can go the whole hog and grow your own (very impressive) sugar crystals from scratch if you've got 6 months to spare (see the incredible results here)! But enough waffling, enjoy this recipe and as always let me know how you get on via instagram and twitter @cakesmyth

Ingredients - makes a tall 6" cake to serve 8-10

For the cakes

  • 295g unsalted butter

  • 295g caster sugar

  • 1 ½ tsp vanilla extract

  • 6 large eggs

  • 290g self-raising flour

  • 74g cocoa powder

  • 150g melted dark chocolate (I like Cadbury Bourneville)

For the icing

  • 250g unsalted butter

  • 1 tsp vanilla extract

  • 500g icing sugar

  • 2 tbsp semi-skimmed milk

  • Black gel food colouring

For the filling

  • 75-100g morello cherry conserve (I use Sainsbury’s Taste the Difference)

For the decoration

  • 200g good quality clear mints

  • Blue gel food colouring, or whatever colour you fancy

  • Gold Lustre Powder (optional)


  • A 6” deep loose-bottomed cake tin, a 6” diameter pyrex bowl and some large non-stick baking sheets, a small offset palette knife is useful, a small food-safe paintbrush (optional - for gold lustre)


  1. Preheat the oven to fan 140C. Grease and line the 6” cake tin with parchment paper and do the same with the pyrex, using 2 thin strips of parchment paper to line it in an X shape to help lift out the cake afterwards.

  2. For the cakes, beat together the butter and sugar until pale and fluffy. Add the eggs one at a time, mixing well in between additions until thoroughly combined along with the vanilla extract. Sieve in the self-raising flour and cocoa powder and mix until just combined. Pour the melted chocolate into a well in the centre and quickly mix until a uniform colour. Weigh the mix evenly between the pyrex bowl and cake tin then bake for 1hr 10 - 1 hr 20 until a skewer poked in the centre of each cake comes out clean (the bowl cake may take slightly longer than the cake tin one). Remove from tin/bowl and set on a wire rack to cool completely.

  3. To make the coloured gems, use a sharp chef’s knife to shatter the mints into small sharp pieces. Divide the mints into groups depending on how many shades you want - I like to do 4 shades, clear, light, medium and dark. For each shade, in a separate small bowl add a drop of food colouring and dilute to the required colour. Take a tbsp of this coloured liquid and quickly mix into the mints until all coated. Quickly drain and transfer to a large lined baking sheet. Place in a very low oven (30 C or lowest fan setting) for a few hours or ideally overnight to dry out completely. Store in an airtight container until ready to decorate.

  4. For the icing, beat the butter and vanilla extract until light and creamy. Sieve the icing sugar into a large bowl then add one tbsp at a time with the mixer on low speed. This will help you avoid a grainy buttercream. Once all the sugar has been added, increase the speed and beat for a further minute, adding the milk to loosen the icing so it is smooth and spreadable. Reserve ¼ of the icing uncoloured and cover with cling film until used. Use the black food colouring to create a grey icing with the remainder and set aside.

  5. To build the cake, level both the sponges using a large bread knife and keep the offcuts. Sandwich together in a mound shape with a generous layer of the cherry jam. Use a small palette knife to generously cover the outside of the cake in grey buttercream, smooth it then put in the fridge for 10 minutes to firm up. Use a sharp paring knife to carve out a cavity in the cake that will be your geode. Use the uncoloured buttercream to generously coat the inside of this cavity to act as glue. Press your gems into the cavity with the darkest colour in the middle getting lighter towards the outside. Some small clumps of gems can work really nicely here and you can plug the gaps with individual pieces.

  6. To finish, dust the gems and geode with a little gold lustre to give a pearlescent effect. Crumble the offcut sponge and spoon around the base as a chocolate soil. Enjoy!

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