In a medium bowl, combine the flour, 1/2 cup softened butter, sugar, and salt. Mix until a crumbly dough forms, then press it firmly and evenly into the bottom and up the sides of your tart pan.

Prick the base of the crust with a fork several times and bake for 18 to 20 minutes, or until the edges are lightly golden. Remove and let cool completely on a wire rack.

To make the caramel, combine the condensed milk, 4 tablespoons of butter, and the golden syrup in a medium saucepan over medium-low heat.

Whisk constantly for about 8 to 10 minutes. The mixture will darken to a pale golden brown and thicken significantly.

Pour the hot caramel over the cooled tart base, spreading it evenly with a spatula. Allow it to set in the refrigerator for at least 30 minutes.

Place the chopped dark chocolate in a heat-proof bowl. Heat the heavy cream in a small saucepan until it just begins to simmer, then pour it directly over the chocolate.

Let the cream and chocolate sit for 2 minutes, then whisk gently until smooth and glossy.

Pour the chocolate ganache over the set caramel layer. Tilt the tart pan slightly to ensure an even coating.

Refrigerate for at least 1 hour before removing the sides of the pan and slicing to serve.