Begin by roasting the chocolate. Preheat the oven to 120 celsius (250 Fahrenheit) and add the chocolate into a small baking dish and place in the oven for 60 minutes, stirring every 10 minutes, until a deep caramel colour. The chocolate should be smooth and velvety. If it it's grainy press through a fine mesh sieve until smooth. Set aside to cool slightly.
Whisk the cream and condensed milk together in a stand mixer until soft peaks form. Fold through half the white chocolate and mix well. Pour half the mixture into a rectangle ice cream container or loaf tin and add half of the remaining chocolate and use a skewer to swirl through the ice cream. Top with remaining ice cream and swirl in the remaining caramelised white chocolate. Cover with cling film and freeze for 6 hours or overnight before serving.