Line a 1 litre pudding basin with two layers of cling film.
Press ice cream into the basin, cover, and return to the freezer.
Preheat oven to 180°C and line a 20cm cake tin.
Melt 50g chocolate in a bowl over simmering water and set aside.
Cream 125g butter and 125g sugar until fluffy, then beat in eggs.
Stir in melted chocolate and fold in 125g flour.
Bake batter for 25 to 30 minutes until a skewer comes out clean.
Increase oven temperature to 220°C.
Cool cake in the tin for 10 minutes, then move to a wire rack.
Whisk egg whites to soft peaks.
Whisk in half the caster sugar and cream of tartar to stiff peaks, then fold in remaining sugar.
Boil 200ml cream, stir in 200g chocolate and optional alcohol for the sauce.
Place the cooled cake onto a flat baking tray.
Unmold the ice cream and place it upside down onto the cake.
Cover the cake and ice cream quickly with thick peaks of meringue.
Bake for 3 to 4 minutes until meringue is set and golden.
Transfer carefully to a serving plate using a palette knife.
Serve immediately with hot chocolate sauce.