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  • Writer's pictureAndrew Smyth

Prism Cake for #LGBTSTEM day

Updated: Aug 23, 2021

#LGBTSTEMday is occurring for the second time this Friday 5th July, and its purpose is to raise the profile and celebrate LGBT people working in STEM fields. I teamed up with Yan (who you might not know works at The Crick as a biologist) to create a recipe to celebrate the occasion. Mixing a classic rainbow cake with a bit of STEM we’ve arrived at a a rainbow PRISM cake, reflecting the combination of LGBT in STEM! If you’d prefer a regular rainbow stacked circular cake, just avoid the trimming steps and increase the buttercream quantities by 50%! Check out the Youtube video before following the recipe below :) Enjoy, get involved and let me know how you get on using the hashtag #LGBTSTEMday , I can't wait to see your bakes! Andrew x

Ingredients - Makes a fabulous sharing cake

For the cake

  • 750g salted butter at room temperature, plus a little extra for lining tins

  • 750g caster sugar

  • Zest 3 lemons

  • 2 tsp vanilla extract

  • 9 large eggs

  • 750g self-raising flour, plus a little extra for lining tins

  • 100g ground almonds (we used Yan’s Chinese almonds which were INCREDIBLE and help keep it moist)

  • Various gel food colours (Wilton or Americolor are best for vibrant, liquid gels will give you a more pastel shade and may change colour in the oven. The colours we used were hot pink, orange, sunset yellow, green, blue and a mix of the pink and blue to create purple.)

For the icing

  • 250g salted butter, soft at room temperature

  • 500g icing sugar, sifted

  • 1 tsp vanilla extract

  • 2 tsp lemon juice


  • 3 20cm diameter cake tins and an electric Hand Whisk


  1. Start by greasing each tin with a little butter then tossing some extra self-raising flour around them to coat in a thin layer, tipping out the excess. Preheat the oven to fan 160 C (320F) / conventional 180 C.

  2. In a large mixing bowl (very large: if you don’t have one then do the mixing in 2 batches), add the soft butter and caster sugar and beat using an electric whisk for about 5 minutes until pale and fluffy. Add the lemon zest and vanilla extract and mix to combine.

  3. Add the eggs, one at a time, whisking until each one is well incorporated. Regularly scrape down the sides of the bowl using a spatula. Sift in the self-raising flour and add the almonds and gently fold to combine the mix together. You want to mix to be “dollopable”, so add a few extra tablespoons of milk to loosen it up if needed.

  4. Split the mix evenly into 6 bowls, I find the easiest way is to use a new bowl to weigh the whole mixture, then divide this by 6 and weigh each bowl individually - this will help you get nice even layers. Add the desired amount of colour (if using gels, I dip the handle of a teaspoon in and take a scoop - this should be about right) to each bowl and mix well to create 6 rainbow-coloured batters.

  5. We have 3 tins, so are able to bake in 2 batches, washing the tins in between. If you’ve got more or less, just adjust your batch baking accordingly. Pour 3 of the colours into their respective tins (one colour per tin) and spread flat using a small offset palette knife or spatula.

  6. Depending on your oven size, you can bake the cakes in batches - most ovens should be able to take 3 at once between the mid and top shelves. Bake for 20-25 mins until the edges are pulling away from the sides, the top is springy to the touch and a skewer inserted into the middle comes out clean. Leave to cool on a wire rack for 5 minutes before running a knife around the edge and flipping over onto the wire rack. The cakes should come free, leave them on the rack to cool completely.

  7. Whilst the cakes cool, time for the buttercream. If you’re feeling adventurous, you could go for a swiss meringue buttercream, we went for a classic American buttercream for simplicity. Add the soft butter (it should be easy to push your finger into it) to a stand mixer or large bowl and beat for 5 minutes until very light in colour and even softer. With the mixer on a medium speed, add the sifted icing sugar one tablespoon at a time to the mix, waiting for it to incorporate before adding the next tablespoon. Once it’s all been added, whip the mixer up to high speed for at least 5 minutes until all the sugar dissolves. Add the vanilla extract and lemon juice and beat until uniform. Taste and if you’d like it a little less sweet, add a little more of the acid (lemon juice).

  8. Take each of the sponges and level the tops with a bread knife (or cake leveller if you’re a fancy type) so they are nice and flat. Stack together dry and cut the stack into a triangular shape, like a prism. I made a dodgy triangle template from a card folder to cut around for this, feel free to get a bit more precise. Save the offcuts for decoration/cake pops.

  9. Spread a thin layer of the icing between each layer of cake before adding the next layer. Chill in the fridge for 5 minutes before adding each layer if you have the time, to keep it stable. Once the top layer is on, crumb coat with a thin layer of icing all the way round. The idea of this layer is to catch all the loose crumbs and bond them to teh cake, so the final layer looks clean and white. Chill for at least 30 minutes before adding this final layer, making it as neat as possible with a palette knife or whatever you have to hand!

  10. Decorate the top with the crumbs from the leftover cake offcuts. We did this to look like a prism splitting light on top of the cake but feel free to let your imagination run wild. Enjoy and happy #LGBTSTEMday !

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