Prepare the pan: Line an 8x8-inch baking pan with parchment paper, leaving an overhang on two sides. Lightly spray with cooking spray.
Combine: Add condensed milk, brown sugar, butter, golden syrup, and salt to a large heavy saucepan over low heat. Stir constantly until butter melts and sugar dissolves fully.
Cook: Increase to medium heat. Stir and cook 3 to 5 minutes until thickened and golden caramel in color.
Add chocolate: Remove from heat. Add white chocolate melts and stir until completely smooth.
Pour: Pour into prepared pan. Smooth the top. Sprinkle with flaky sea salt if using.
Chill: Cool to room temperature, then refrigerate uncovered for 1 hour, cover, and chill at least 3 more hours or overnight.
Slice: Lift out using parchment overhang. Cut into 24 squares with a sharp knife.