In a stand mixer with the paddle attachment or using electric beaters, cream the butter and sugar together until light and fluffy. This should take 2-3 minutes for the sugar to absorb and not feel gritty when you rub some between your fingers.
Meanwhile, coarsely grind the fennel seeds in a mortar and pestle or spice grinder, scoop out half of the coarsely ground seeds and reserve. Then grind the rest until it’s a fine powder, reserving a quarter of the finest ground seeds for the sugar coating.
To the creamed butter and sugar, add the coarsely and finely ground seeds (besides the sugar coating ¼) along with the vanilla and salt flakes and then beat for another 30 seconds. Sprinkle in the plain flour and almond meal and beat on a medium speed to combine for a minute or so until the dough is a uniform colour, stopping and scraping down the bowl to incorporate halfway through.
Dust a little flour over a clean bench, then turn out the dough and shape it into a smooth ball. Don’t add too much flour - just enough to lightly coat the bench and stop it from sticking. Flatten the dough into a disc and pop in a container and into the fridge for 10-20 minutes - or overnight if making ahead of time.
When you’re ready to bake, preheat the oven to 170℃ (150℃ fan). Line a large baking tray (light-coloured, if possible), or two smaller ones.
Cut the chilled dough into four equal parts. Working one piece at a time, dust your clean bench with a little more flour and roll into a 2cm diameter log. Use a sharp knife to cut 2cm discs all the way along the log. Pop onto your lined tray and gently squash down to form a flatter round ‘coin’ about 1cm high. Repeat until all of the dough is finished.
Pop into the preheat oven for 15 minutes, turning the tray around halfway through in case your oven has any hot-spots. They’ll be lightly golden on top when ready. Pull out and leave them to cool on the tray for 5 or so minutes.
While the biscuits cool, pop the caster sugar and reserved ground fennel seed onto a flat plate or tray with a lip and mix together with your fingertips. When biscuits have cooled and firmed up, toss each in the fennel sugar mix, a few at a time, then transfer to another tray or container without a lid.
Once completely cooled, store in an airtight container or pop into gifting jars for your lucky recipients.