Combine all the ingredients for the lemon curd in a large bowl.
Set the bowl over a pan of barely simmering water and whisk continuously until the butter has melted.
Continue stirring until the curd has thickened (the curd will easily coat the back of a spoon), then pour into an airtight container or jar and allow to cool completely.
The lemon curd will last for up to 2 weeks in the refrigerator.
For the cake
Preheat the oven to 180°C/350°F.
Sift the flour, baking powder and salt into a medium mixing bowl.
Heat the milk and vanilla in a saucepan until hot, taking care not to allow the mixture to boil.
In the bowl of a stand mixer fitted with the whisk attachment whip the eggs with the sugar for 4-5 minutes until light and fluffy.
The eggs will have tripled in volume. Slowly pour in the oil and allow to whip into the egg mixture.
Stir the dry ingredients and the milk, alternatively, until the batter is smooth and creamy. Stir in the lemon zest.
Pour the batter into lined and greased cake pans (approximately 20cm/8inch).
Place in the oven and allow to bake for 40-45 minutes until the cakes are golden brown and a toothpick inserted comes out clean.
Remove from the oven and allow to chill completely.
Make the whipped cream
Whip the cream with the icing sugar until it stiff peaks form.
Assemble
Sandwich the cooled cake layers with the whipped cream and lemon curd then serve.