Start by mixing the butter and caster sugar together until well combined and creamy looking. This is best done in a stand mixer with the paddle attachment for 3-5 minutes
Add the egg and vanilla extract, and mix until fully incorporated
Add the plain flour, cocoa powder and salt, and mix until combined and it starts to clump together
Bring the mixture together into a smooth dough with your hands, and form into a rough disc shape. Wrap the dough in cling film and put it in the fridge to chill for 30 minutes
Meanwhile, pre-heat your oven to 160C Fan/350F/Gas Mark 4, and line a few baking trays with baking paper
Using a rolling pin, roll the dough out onto a floured surface to a thickness of 5mm. Keep turning the dough as you roll, to ensure it doesn't stick to the surface, and add more flour underneath as needed
Cut out your biscuit shapes and place them onto the lined baking trays. You will need to re-roll out the dough to use it all up
Bake the biscuits for 10-12 minutes. Leave to cool on the trays for at least 10 minutes before moving to cooling racks to cool fully
Decorate the biscuits however you like, or leave them plain
Store in an airtight container in a cool place for 3-4 days (although they can last up to a week), you can also freeze them
Notes
Make sure all your ingredients (particularly the butter and egg) are at room temperature before you start baking.
I prefer to use unsalted butter for this recipe, you can also use a baking spread but you may need to add a little extra flour due to the higher water content in baking spreads.
You can use any cocoa powder for this recipe, of course the better quality it is, the better flavour it will have. Also do make sure you are using cocoa powder and not a hot chocolate powder.
For the best flavour, make sure to use a good quality vanilla extract and not an essence. You can also use a vanilla bean paste. Or to add some flavour to the biscuits, you could use and orange or peppermint extract for example.
Although I provide cup measurements, I highly recommend weighing your ingredients out using digital kitchen scales. It is the most accurate way to measure ingredients and will ensure the best results. Digital scales are very low cost and can be purchased for around £12 ($16.50).
For teaspoon (tsp) and tablespoon (tbsp) measurements, please use measuring spoons and not the type of spoons you eat with. Again this will ensure accuracy and provide the best results.