Preheat the oven to 200 degrees C (fan-assisted) and line a muffin tin with cake cases.
Toss together the flour, bicarb, and sugars gently until well mixed.
In another bowl, whisk together the eggs, buttermilk, oil, and vanilla for a few minutes.
Pour these wet ingredients into the dry ingredients and mix together until the batter is smooth (around 2 minutes with an electric whisk).
Add the lemon juice and zest, and whisk for another minute.
Spoon 2 tablespoons of muffin mix into each cupcake case.
Add a tablespoon of lemon curd.
Top with another tablespoon of muffin mix.
Sprinkle on a little granulated sugar (optional).
Bake at 200 degrees for 10 minutes, before reducing the temperature to 160 for 10-15 minutes, checking after 10.
Notes
If you don’t have buttermilk, add 2tbsp lemon juice to normal milk and leave it on the side until it’s at room temp.
Muffins are ready when the tops are golden brown and domed and still bounce back a little when pressed. You can also use a skewer to check that the centre is cooked, trying your best to avoid lemon curd.