Melt the chocolate in a bowl suspended over a pan of hot, but not boiling water. Leave to cool a little.
Preheat the oven to 180℃ (160℃ fan, 350℉, Gas 4).
Cream the butter and sugar together until light in colour and fluffy in texture.
Beat in the eggs, one by one, followed by the orange zest and melted chocolate.
Sift in the dry ingredients: flour, baking powder, bicarbonate of soda, cocoa powder and salt. Stir until nearly combined.
Reserve 1 tbsp of the squeezed orange juice for the rhubarb, then measure out 50 ml. If there's not enough juice, top up with water.
Stir in the orange juice and yoghurt until everything is just combined.
Divide the batter between two round 20 cm (8 inch) silicone moulds or lined cake tins. Bake in the middle of the oven for thirty minutes or until the tops are firm to the touch and an inserted skewer comes out more or less clean.
Leave to cool for ten minutes, then turn out onto a wire rack to cool completely.
Meanwhile stew the rhubarb with the sugar and the reserved 1 tbsp of orange juice until the rhubarb is soft and most of the water has evaporated. The mixture should be fairly thick and will take between about ten minutes. Leave to cool.
Stir in the orange liqueur, if using.
Whip the cream into peaks, then fold into the rhubarb mixture.
Sandwich the cooked sponges with the cream, then dust the top of the cake with icing sugar.
Please note: calories and other nutritional information are per serving. They’re approximate and will depend on serving size and exact ingredients used.