Grease a 20cm round springform cake tin with oil and line the sides with non-stick baking paper. Melt the chocolate for the ice cream in a large heatproof bowl set over a pan of simmering water or in 30-second blasts in the microwave, stirring regularly. Set aside to cool.
To make the base, melt the butter in a medium saucepan on the hob or in a bowl in the microwave. Blend the cookies in a food processor or seal them in a ziplock bag and bash with a rolling pin until you have fine crumbs. Mix the crumbs into the melted butter until well coated and spread evenly in the base of the tin with the back of a spoon. Pop into the freezer for at least 15 minutes to set.
Returning to the ice cream, once the melted chocolate is cool, add the condensed milk and mix until thick and stiff. Using an electric whisk, beat the cream in a separate bowl until stiff too. Fold the whipped cream into the chocolate mixture until well blended.
Pour this mixture onto the set cake base, levelling the top with the back of a spoon. Arrange your chosen decorations on top. Freeze for at least 8 hours or overnight, until frozen solid.
This will keep for up to three months in the freezer. Remove the cake 10 to 20 minutes before serving, depending on how warm your room is, until it’s softened a little. Drizzle the caramel over, if using. Cut the cake into eight pieces. This is best done with a long sharp knife that has been dipped in just boiled water and wiped dry between each cut.