Melt the butter, chocolate and sugar in a small saucepan over low heat. Allow to cool a little.
Sift the dry ingredients into a bowl and make a well in the centre.
Break the egg into the centre and start mixing adding the milk as you go. Add the chocolate mixture and stir well.
Spoon the batter into a prepared 20 cm (8″) cake tin and scatter the blackcurrants over the top.
Rub the remaining butter, sugar, flour and cinnamon together until the mixture resembles breadcrumbs. Scatter this over the top of the blackcurrants.
Bake in a preheated oven at 180℃ (350℉, Gas 4) for about 45 minutes until well risen and a skewer inserted comes out clean.
Serve warm with cream or custard for pudding or at room temperature for afternoon tea.Please note: calories and other nutritional information are per serving. They're approximate and will depend on serving size and exact ingredients used.