Preheat oven to 160deg C (fan)/180 deg C/350 deg F/gas mark 4
Line a 12 hole muffin tray with cupcake cases.
In a large bowl, cream the butter and sugar together until fluffy. Add the eggs, one at a time, to the creamed butter and sugar and beat well.
In a jug, combine the milk, lemon juice and sour cream. Alternate adding the flours and milk to creamed butter mix as follows - add one-third of the flour and mix well. Add half the milk mixture and mix well. Add another third of flour and mix well. Add the reaminder of the milk and mix well. Finish by adding the last third of flour and the lemon zest and beat well.
If the mixture looks like it is curdling slightly, donít panic Ė simply add another spoonful of plain flour and beat well.
Spoon out the mixture evenly between the cases and bake in the oven for 25 minutes. Use a cake skewer in the centre of one the cupcakes to check they are cooked through. They will appear quite a light golden brown colour even when cooked.
Leave in the tin for 10 minutes and the place carefully on a wire rack to cool completely.
When the cupcakes are completely cold, they can be iced with the fresh lemon buttercream and decorated with a sugared lemon slice (pots of these are sold in most supermarkets) or some grated peel from a lemon.
Serve immediately or store in an airtight container for up to 3 days at room temperature.
For the buttercream:
In a large bowl, beat the butter, milk, lemon juice, lemon rind and half the icing sugar until smooth. Gradually add the remainder of the icing sugar and beat until thick and creamy. This can take several minutes.
Use this icing on the lemon cupcakes or to ice the middle and top of the lemon layer cake.
Store any unused icing in an airtight container at room temperature for up to 3 days. Make sure to beat well before re-using.