Preheat oven to 160° C (fan-forced). Line a 12 hole muffin tin with muffin cases.
Place the self raising flour into the Thermomix bowl. Press Turbo 5-10 times to sift. Remove from the bowl and set aside until needed.
Place the butter and sugar into the TM bowl. Mix for 30 seconds, Speed 4. Scrape down the bowl and repeat for a further 2 times, until pale and creamy.
Reduce to Speed 2 and add the eggs one at a time through the MC hole while the blades are turning.
Replace the MC and mix for a further 20 seconds, Speed 5. Scrape down the sides of the bowl.
Add the vanilla extract, milk and orange juice and mix for 20 seconds, Speed 4.
Add the self raising flour, poppyseeds and orange zest and mix for 15 seconds, Speed 5 (using the spatula through the MC hole to help mix).
Spoon into the prepared muffin cases.
Bake for 20-25 minutes or until they spring back when touched lightly in the centre. Allow to cool for 5 minutes before transferring to a wire rack. Place wire rack over a tray to catch any syrup that may drip.
While the muffins are still warm, poke holes with a skewer onto the top of each (about 5 holes in each).
Place the orange juice and caster sugar into a microwave-safe jug or bowl. Cook on 30 second bursts for 3 minutes, stirring to dissolve in between bursts.
Spoon the syrup over the top of the muffins. Leave to cool completely.