Preheat oven to 170°C and line a 20cm square baking pan with parchment paper.

In a large mixing bowl, whisk together 200g melted butter and sugar. Add eggs and vanilla extract, whisking until the mixture is glossy and well emulsified.

Sift the cocoa powder, gluten-free flour, and salt into the wet ingredients. Fold the mixture gently with a spatula until just combined.

Pour the brownie batter into the prepared pan and bake for 20 minutes.

Prepare the caramel by combining the condensed milk, 50g butter, and golden syrup in a heavy-based saucepan over medium-low heat.

Whisk the caramel continuously for 8 to 10 minutes until thickened and pale golden.

Pour the hot caramel over the partially baked brownie base and return the pan to the oven for an additional 12 to 15 minutes.

Remove from the oven and allow the pan to cool completely at room temperature, then refrigerate for at least 2 hours to set the caramel.

Melt the chopped dark chocolate and 30g butter together using a double boiler or microwave.

Spread the chocolate topping over the chilled caramel layer and refrigerate for 30 minutes before slicing into squares.