Zest the organic lemon, set aside.
In a heatproof glass bowl, cream butter and sugar together. Do not use metal bowls as it will react to the acidity in the lemon juice.
Add egg and beat until incorporated.
Add lemon juice and beat until incorporated.
Place over a pot of boiling water. Do not let the bottom touch the water.
Whisk gently to keep the mixture moving as it heats up.
Occasionally, check the temperature of the mixture. You should see the curd thicken up and it will coat the back of your spoon. Allow the mixture to reach 180F-185F. It might take a while (10-15 min), so be patient.
Take the mixture off the heat, add the lemon zest into the hot mixture to release the oils and mix until combined.
Pour through a sieve into another bowl for a smooth curd.
Let it cool down then place in the fridge. It will set up into an even firmer and thicker curd after you've chilled it in the fridge.
Place egg whites and sugar into a heat proof bowl. Stainless steel is light and safe.
Keep 200 grams of the buttercream for use in our current recipe. Freeze the rest of the buttercream for use at a later date.
Add 8 tablespoon of the curd into the buttercream, a tablespoon at a time and beat until incorporated.
Pipe a ring of lemon buttercream as a dam then fill it with the lemon curd. Assemble the shells together, place in fridge to mature for 24 hours. Enjoy!