Method
Preheat your oven to 140°C (275°F) / fan 120°C (250°F) / gas mark 1.
Line two large baking trays with baking parchment.
To make the biscuits, combine the flour, bicarbonate of soda, sugar, raisins, and salt in a large mixing bowl or the bowl of a stand mixer fitted with the beater attachment.
Add the softened butter and golden syrup and mix until everything is thoroughly combined into a sticky dough.
Divide the dough into 16 equal balls (about 1 heaped tablespoon each).
Place 8 balls on each prepared baking tray, leaving plenty of space between them as they will spread.
Bake for 25–30 minutes, turning the trays halfway through for an even bake, until the biscuits are a beautiful golden brown.
Let them cool completely on the trays before handling.
To make the buttercream, beat the softened butter in a large bowl until light and fluffy. Very gradually, add the rum a little at a time, beating continuously.
Add the sifted icing sugar and beat again on a low speed until just combined. Be careful not to overwork the buttercream, as it can split easily.
Once the biscuits are completely cool, carefully lift them with a palette knife.
Spread a thick layer of the rum buttercream on the flat side of 8 biscuits, then gently sandwich them together with the remaining 8 biscuits.