Yummy. I remember this dessert from my Aunt in Scotland. It's one of my favorites!
Swapping the traditional plain digestive biscuits for a gluten-free shortbread fundamentally alters the structural integrity of this confection, requiring a firmer hand when pressing the crumbs into the pan to prevent the final slab from crumbling apart upon slicing. Originating in nineteenth-century British tradition as a cold-set refrigerator cake that required no baking, tiffin relied on pantry staples like golden syrup and crushed biscuits to create a dense, sliceable treat. When melting the butter, sugar, and syrup mixture together, bring it to a gentle boil for precisely two to three minutes so the liquid reduces enough to bind the dry components without leaving the base soggy. Crushing the biscuits inside a sealed plastic bag using a rolling pin provides the ideal mechanical control needed to achieve uniform crumbs rather than irregular chunks. Melt the top chocolate layer gently over indirect heat, ensuring zero water contact seizes the cocoa solids, before transferring the Swiss roll tin to a refrigerator maintained at four degrees Celsius to set the fats properly.