Heat the oven to 190oC/375oF/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.
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 tops.
If anyone can't get hold of Lemon Curd at your local store, you can make your own. Below I have included a recipe but feel free to use your own recipe.
The following will make a single large jar of lemon curd. Obviously you can multiply up the quantities to make more.
Put a large bowl over a pan of water.
Put this on the heat, and once the water has come to the boil put 55g (2oz) of unsalted butter and 225g (8oz) of sugar into the bowl.
Meanwhile, grate the rind of two lemons and squeeze their juice into another bowl.
Once the butter has fully melted, add the rind and juice to your large bowl over the pan. 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. Over the next 20 to 30 minutes it will thicken and you may start to see some white parts forming in the otherwise yellow mixture. This is perfectly normal. 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 using a funnel if necessary to help you get it in, without it running down the sides.
Seal the jar so that it is airtight and store the curd in the fridge. It should keep for around a month.