Firstly, make the Lemon Curd.

1. Add caster sugar, cornflour, water and lemon juice to small pan. Stir ingredients with a metal spoon until all well combined.
2. Cook over medium heat, stirring with a metal spoon constantly. Do not let mixture stick to the bottom of the pan or burn.
3. Mixture will begin to thicken up after a few minutes.
4. Continue to stir until mixture thickens and becomes smooth.
5. Add the butter and whisked egg yolk and cook for three minutes until all well combined.
6. Do not let the curd stick to the pan or burn. Continue to stir through.
7. Add the lemon rind and stir through until smooth and well combined.
8. Set aside and refrigerate immediately. The lemon curd will harden in the fridge, therefore will need to bring to room temperature to soften it and be able to mix it with the Swiss Meringue Buttercream at the final stage of this recipe.

Secondly, make the Swiss Meringue Buttercream.

1. Fill a medium sized saucepan with about 2 centimeters of water. Bring to a boil then reduce to medium heat.
2. Place egg whites and caster sugar in a clean heat proof  stand mixer bowl. Make sure bowl is completely free of grease. If in doubt wipe with white vinegar or lemon juice. Whisk together until all incorporated, then place bowl on top of sauce pan. Make sure the bowl does not touch the water.
3. Whisk and mix ingredients until sugar dissolves completely. Scrape the bowl on the sides with a silicone spatula so that all the sugar dissolves with the egg whites.
4. Temperature should reach 70 degrees Celsius (158 Fahrenheit). If you do not have a food thermometer you can rub the mixture between your fingers and you should not feel any sugar granules. The mixture will also change and become runny and foamy. (You can watch how it changes on my Step by Step video above).
5. Transfer bowl to stand mixer and mix at medium-high speed until the meringue starts to come together. It should become thick and glossy and increase in volume. The meringue should not be runny or fall from an upside down spoon.
6. Transfer meringue into another large bowl. Set aside until it has completely cooled to room temperature.
7. Wash mixer bowl and add unsalted butter and mix at high speed until butter changes colour to a very pale cream and it increases in volume. It can take up to 10 minutes or more. You must scrape the sides off the bowl to ensure all butter is beaten.
8. Once butter reaches a very pale consistency, mix in the meringue and mix together from medium to highest speed for 5 minutes.
9. Scrape the bowl and set aside.

Third, and last step is to combine the completely cooled Lemon curd and Swiss Meringue Buttercream.

1. Once the Swiss Meringue Buttercream is complete and the Lemon Curd has completely cooled down to room temperature. You will add the lemon curd into the mixer bowl that has the finished Swiss Meringue Buttercream.
2. Combine and mix both the Lemon Curd and Swiss Meringue Buttercream in the mixer bowl.
3. Scrape the bowl once or as necessary to ensure both are well combined.
4. At this stage you may choose to add food colouring of your choice. I have opted to add yellow gel food colouring as shown in the how to video above. You may also leave the frosting in its natural pale ivory colour.