Prepare a large baking tray by lining with good-quality non-stick baking paper.

In a smallish bowl, weigh and mix together the flour, custard powder, cocoa, xanthan gum and salt and stir well to blend. Set aside.

In a larger bowl, cream together the butter, sugar, golden syrup and vanilla extract until light, pale and smooth. This is easiest with an electric whisk.

Gradually add and beat in the dry mix (about a quarter at a time) until well blended and the dough starts to come together. If making a dairy free/vegan version, it will help to add ½ to 1 tbsp of DF milk at this point to support binding and to add extra lightness to the dough. This is NOT necessary if making with dairy butter.

Work the dough into an even, smooth block using a firm mixing spoon or spatula.

If using a Bourbon Biscuit mould, very lightly dust the surface with flour.

Use a flat butter knife to transfer about a teaspoon of biscuit dough into the mould and spread roughly-evenly.

Place the mould against a piece of baking paper on a firm surface and push down reasonably firmly on the plunger for a few seconds to even out the dough and ensure a good imprint.

Push and gently 'shake' the moulded biscuit out onto the prepared baking tray. Each biscuit should be about ¾ cm in depth. If too thick or thin, adjust the amount of dough added to the mould (until you find a happy point) and re-use the dough of any biscuits you are unhappy with.

Repeat the process of dusting, filling and moulding for all the dough.

If you are not using a mould, place the dough on a piece of baking paper very lightly dusted with flour and lightly dust the dough top.

Carefully roll the dough to a thickness of about ¾ cm (if you want a pattern, use an embossed rolling pin).

Cut the cookies into equal oblong biscuit shapes using a cookie cutter or sharp knife.

Using a fine skewer tip, poke a line of holes down each long side of each cut biscuit dough.

Carefully transfer to the prepared baking sheet.

Repeat the process, bringing any remaining dough back together until all the dough has been used. Try not to add too much flour or excessively work the dough.

Place the tray of dough biscuits in the fridge for about 20 minutes to chill, or alternatively in the freezer for about 10 minutes.

Meanwhile, pre-heat the oven to 180 C/350 F/Gas 4.

When the biscuits have chilled, rearrange on the tray if necessary to space evenly.

Bake the biscuits for 12 to 14 minutes until just starting to darken slightly at the edges.

Remove from the oven and leave on the baking tray to cool completely.

In a medium to large bowl, beat the softened butter until light, pale and smooth.

Gradually add the icing sugar, custard powder, cocoa, coffee (if using) and vanilla and beat through until smooth and creamy.

To sandwich the biscuits into Bourbon Creams, pair the biscuits.

Take one biscuit from each pair and spread a generous dollop of buttercream across the surface (making sure the pattern is on the outer side).

Sandwich with its biscuit pair and gently push together.

For a neat effect, use a chopstick or small rounded fondant tool to scrape any excess buttercream from the sides of each biscuit.

Store in an airtight container at room temperature. Will stay fresh for at least a week.