Preheat oven to 200° C (fan-forced) or 390 degrees fahrenheit. Line a 12 hole muffin tin with muffin cases.
Place the plain flour, salt, bicarbonate of soda and baking powder 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 Thermomix 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, greek yoghurt and lemon juice and mix for 20 seconds, Speed 4.
Add the flour mix, poppyseeds and lemon 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 15 minutes or until they spring back when touched lightly in the centre. Allow to cool for 5 minutes before transferring to a wire rack to cool completely.
To make the lemon glaze, mix the icing sugar with the lemon juice until smooth. Drizzle over the top of the cooled muffins.