Chocolate Biscuit Cake Squares

1. Prepare the Pan and Biscuits

1. Grease a 20x20 cm square baking tin lightly with butter and line it with baking paper, leaving an overhang on the sides for easy lifting later.
2. Place the biscuits into a large bowl and crush them roughly by hand. Aim for a mix of pea-sized rubble and larger crunchy chunks; do not grind them into a fine powder.

2. Melt the Chocolate Base

1. Set a heatproof bowl over a small saucepan of gently simmering water, ensuring the bottom of the bowl does not touch the water.
2. Add the chopped dark chocolate, milk chocolate, cubed butter, golden syrup, vanilla extract, and fine salt to the bowl.
3. Stir occasionally with a spatula over low heat until completely melted, silky, and combined. Remove the bowl from the heat and let it cool for 3 to 4 minutes.

3. Combine and Press

1. Reserve about 3 tablespoons of the melted chocolate mixture in a separate small bowl to create a smooth top later.
2. Tip the broken biscuits into the main bowl of melted chocolate. Fold thoroughly with a silicone spatula until every biscuit piece is fully coated in glossy chocolate.
3. Transfer the mixture into the prepared tin. Press it firmly into the base and corners using the back of a spoon or a flat measuring cup to pack it tightly.
4. Pour the reserved melted chocolate over the top and tilt or smooth with a spatula to create an even, satiny surface. Scatter a few flakes of sea salt across the top if desired.

4. Chill and Slice

1. Refrigerate the tin for at least 2 hours (or until completely set and firm).
2. Use the baking paper overhang to lift the solid block out onto a cutting board.
3. Dip a sharp chef's knife into hot water, wipe dry, and slice the slab into 16 even squares.