Preheat oven to 180°C (350°F) and line a 20cm square baking tin with parchment paper.
Combine 175g melted butter, light brown sugar, flour, ground ginger, cinnamon, and 1 tbsp golden syrup in a bowl until a cohesive dough forms.
Press the dough firmly into the base of the prepared tin and bake for 15 minutes until set; remove and allow to cool slightly.
In a heavy-bottomed saucepan, combine 150g butter, condensed milk, 4 tbsp golden syrup, and sea salt over medium-low heat.
Whisk the caramel mixture constantly for 10 to 12 minutes until it thickens and achieves a light amber color.
Pour the caramel over the baked gingerbread base and return the tin to the oven for 8 minutes to further set the caramel.
Remove from oven and allow the base and caramel layers to cool completely at room temperature.
Melt the dark chocolate and 20g butter together using a double boiler or microwave in 30-second intervals until smooth.
Spread the chocolate topping evenly over the cooled caramel layer using an offset spatula.
Refrigerate the tray for at least 2 hours until firm before slicing into 16 squares.