Prep the tin. Line an 8-inch (20 cm) round springform pan or a 9x5-inch (23x13 cm) loaf pan with parchment, leaving overhang for easy lifting. Lightly grease the sides so the paper sticks.

Break the biscuits. Place biscuits in a large bowl and break them into rough pieces about the size of a thumb. Keep some smaller crumbs for texture. Don’t crush them into dust.

Melt the base. In a heatproof bowl, add dark chocolate, milk chocolate, butter, and golden syrup. Set over a pan of barely simmering water (or use short microwave bursts). Stir until smooth and glossy. Remove from heat and stir in vanilla and salt.

Combine. Pour two-thirds of the chocolate mixture over the broken biscuits. Fold gently until most pieces are coated. If using nuts, dried fruit, or marshmallows, fold them in now. Add more chocolate mixture as needed so everything is lightly but evenly coated. You don’t want pools at the bottom.

Pack it in. Tip the mixture into the lined pan. Press down firmly with a spatula or the back of a spoon to eliminate gaps. For a smooth top, place a sheet of parchment over the surface and press with your hands.

Chill to set. Refrigerate for at least 3 hours, or until firm. Overnight is even better for clean slices.

Make the ganache. Warm the cream until steaming but not boiling. Pour over the chopped chocolate in a bowl. Let sit 2 minutes, then stir until smooth. Add the butter and stir until glossy.

Finish the cake. Remove the set cake from the pan. Peel away the parchment and set it on a serving plate. Pour the ganache over the top and nudge it to the edges so it gently drips down the sides. Add sprinkles or decorations while the ganache is still soft.

Set again briefly. Chill 20–30 minutes for the ganache to firm up. Slice with a sharp knife warmed in hot water and wiped dry between cuts.