Heat the oven to 190 degrees Centigrade /375 degree Fahrenheit/gas 5.
Line a 12 hole muffin tin with paper cases.
Sieve the flour into a bowl and stir in the sugar.
In a large jug mix together the egg, sunflower oil, milk, and vanilla extract using a fork.
Pour the wet ingredients into the dry and stir gently together until just combined.
Put a teaspoon of mixture in the base of each paper case. Top with 1 tsp lemon curd. Top with the rest of the muffin mixture.
Bake for 10-15 minutes until risen and firm.
For the sugar coating, microwave the lemon curd on high for 1 minute until melted.
Put the sugar into a shallow bowl.
Brush on top of each muffin and dip the tops of each one in sugar to coat the top.
For the lemon curd:
Put a large bowl over a pan of water. Put this on the heat, and once the water has come to boil put unsalted butter and sugar into the bowl.
Once the butter has fully melted, add the rind and lemon juice to the large bowl.
Finally, beat two eggs and pour them into the mixture.
Stir the mixture with a wooden spoon. Keep it moving slowly all the time so that it coats the edges of the bowl. This will allow it to start cooking on the hot glass.
Eventually you will see that the mix has become considerably thicker and you will be able to draw the spoon through it and see the bottom of the bowl. Once you have reached this stage it is ready to be poured into the jar.
Remove the pan from the heat and very carefully ladle the hot mixture into a sterilised jar.