Put the chocolate and butter in a bowl set over a saucepan of gently simmering water and leave to melt.
Whisk the egg whites in a large bowl until stiff, moist-looking peaks are formed, then gradually whisk in half the sugar, a teaspoonful at a time, until thick and glossy (see
Mix the warm chocolate and butter mixture gradually into the egg yolks. Stir in the flour and cinnamon, then loosen the mixture with the measured warm water. Gently fold in a spoonful of the meringue, then fold in the remainder.
Pour the mixture into a greased and base-lined 23 cm (9 inch) springform tin. Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 25–30 minutes until well risen and the top is crusty and the centre only just set. Leave to cool for 2 hours in the tin.
Remove the torte from the tin, discarding the lining paper. Cut into wedges. Softly whip the cream, then top wedges of torte with spoonfuls of cream and a sprinkling of marshmallows.