Combine 150 g plain flour, a pinch of fine salt, and 30 g caster sugar in a large mixing bowl.

Rub 65 g cold cubed unsalted butter into the flour mixture until it resembles fine crumbs.

Pour 30 ml olive oil and 15 ml water into the bowl.

Mix quickly until a smooth dough forms.

Cover the dough with a damp kitchen cloth.

Refrigerate the covered dough for 60 min.

Divide the chilled dough into 9 equal balls.

Heat 30 g caster sugar in a dry saucepan over medium heat until it becomes an amber caramel.

Add 10 g unsalted butter and 70 ml double cream to the caramel.

Cook the caramel mixture over low heat until bubbling.

Add 30 g chopped milk chocolate to the bubbling mixture.

Remove the saucepan from the heat and stir until the chocolate melts completely.

Whisk 10 g cocoa powder, a pinch of flaky sea salt, and 10 g chopped roasted hazelnuts into the chocolate mixture.

Leave the chocolate-hazelnut filling to cool until thick enough to scoop.

Scoop the cooled filling into 9 balls of about 1 tablespoon each.

Freeze the filling balls for 30 min until firm but still shapeable.

Flatten 1 dough ball in the palm of your hand into a thin round.

Place 1 frozen chocolate filling ball in the center of the flattened dough.

Bring the dough edges over the filling and pinch them closed with no holes.

Repeat the shaping process with the remaining dough balls and filling balls.

Freeze the sealed biscuits for at least 120 min, or overnight.

Preheat the oven to 180°C, or 160°C fan.

Bake the frozen biscuits for 18 to 20 min until lightly golden brown.

Cool the baked biscuits on a wire rack for 30 min.

Dust the cooled biscuits with 5 g icing sugar before serving.