Heat your oven to 160°C Fan/180°C/350°F/Gas mark 4.
Add 12 Cupcake or muffin cases to a cupcake or muffin tin.
Beat your softened butter to spread it out a little, then add your caster (superfine) sugar and beat until paste-like and soft, this can take a couple of minutes, but it is important to get right because it beats in air.
In a separate bowl add your egg whites and start beating them on a low setting, gradually turn up the speed until they have transformed into white foam which forms soft peaks, this can take a few minutes to achieve.
Mix together your milk, sour cream and vanilla extract and beat in a quarter of it on a low setting, then beat in a quarter of your flour.
Repeat the above two steps until of the milk mixture and flour have been added. Scrape down the sides and base of the bowl again with your spatula to mix everything in.
Add your egg whites in four lots, beating in on a low setting. Wipe down your bowl with your spatula to ensure everything is fully mixed in.
Give the batter a good mix for 10 seconds on a medium-high setting.
Split the mixture into 3 lots and add white gel colour to one, red to another and blue to the last one, fold the colours in.
Place teaspoonfuls of each coloured batter mixture into your cupcake cases, making sure that they all have the same amount.
Bake in the centre of your oven for 20-25 minutes until lightly golden on top, then leave to cool.
Cut a hole in the top of each cake with a cake corer or sharp knife and fill with strawberry jam.