Preheat your oven to 160 degrees Celsius fan. Line your square baking tin with non-stick baking paper.
Add the flour and butter into a large bowl. Rub them together with your fingertips until fine crumbs form.
Stir in the caster sugar smoothly. Knead the mixture gently until a soft dough forms.
Press the dough firmly into the lined baking tin. Prick the surface all over using a metal fork.
Bake for twenty minutes until pale golden. Remove the tin and let the base cool completely.
Add the caramel butter, muscovado sugar, and condensed milk into a saucepan. Pour in the golden syrup now.
Heat gently until the sugar dissolves completely. Bring the mixture to a boil while stirring constantly.
Simmer for six minutes until thick. The caramel will turn a deep golden fudge color.
Pour the hot caramel over the cooled shortbread base. Spread it evenly and let it set at room temperature.
Melt the chocolate pieces in a heatproof bowl over simmering water. Do not let water touch the chocolate.
Pour the melted chocolate over the set caramel layer. Spread it smoothly to the edges using a spatula.
Leave the tray to set fully in a cool place. Slice into neat squares using a warm sharp knife.