Line an 8-inch by 8-inch (20cm by 20cm) baking tin with baking parchment. Use a square tin if you intend to make shortbread fingers. Use a circular tin in you would prefer to make petticoat tails'.
Put the softened butter into a large mixing bowl. The butter needs to be soft so that it will mix properly with the other ingredients. Don’t be tempted to soften it in the microwave and if you do, be very careful that you do not end up with melted butter. Melted butter will not give you the lovely crumbly results you’re after.
1 cup unsalted butter
Mix the butter well so that it is smooth and creamy. (This step makes adding the sugar easier.)
Add the caster sugar and stir until it is well combined.
2/3 cup caster sugar
Add the semolina and a small pinch of salt small pinch of salt. (Only add a pinch of salt if you've used unsalted butter.) Stir again until everything is distributed through the dough.
1/2 cup semolina, 2 3/8 cups plain white flour
Sieve the flour into the bowl. This is easier if you add half the amount and incorporate that, followed by the rest of the flour. Only mix it enough so that there are no dry flour bits left in the bowl. You do not want to overmix the flour.
2 3/8 cups plain white flour
Put the dough into the prepared baking tin and smooth it down gently using the back of a dessert spoon. Prick the top of the dough with a fork to make the iconic shortbread pattern and place it in the fridge to chill.
At this stage, you may wish to score the top to mark out the ‘fingers’ as a guide for when it’s baked. Or if you've used a circular baking tin, mark the top into triangular wedges to make petticoat tails. Depress all the way around the edge of the shortbread, using your fingers, to make pretty indentations.
Pop this into the fridge for 30 minutes to chill and preheat the oven to 350F/180C/160 C Fan
Sprinkle a little caster sugar over the shortbread for a charming decorative touch.
After 30 minutes, once the oven is at temperature, place the baking tin into the oven and bake for approximately 30 minutes.
It is done when the shortbread is a light golden brown.
Allow the shortbread to cool in the tin for at least 20 minutes before taking it out of the tin and letting it cool on a cooling rack.
Once it is cool cut it into fingers (or petticoat tails if you baked the shortbread in a circular tin) and store in an airtight container.