Preheat your oven to 180 degrees and line a 12 hole muffin tray with paper cases.
Place the softened butter, caster sugar, self-raising flour, orange juice, eggs and vanilla extract into your Thermomix bowl and mix for 10 seconds, speed 5.
Scrape down the sides of the bowl and mix for a further 10 seconds, speed 4 or until the mixture is smooth.
Remove your Thermomix bowl from the machine and add the sultanas, stir them through with your spatula before filling the muffin cases you prepared earlier until they are 2/3 filled.
Place the muffin trays into your pre-heated oven and bake for 18 - 20 minutes or until cooked when tested with a skewer. Allow the cakes to cool in the tins for 5 minutes before carefully transferring to a wire rack to cool completely.