Preheat the oven to 180°C. Line a cupcake baking tin with paper cases.
Cream the butter and brown sugar together in a mixing bowl. Add the egg yolk, beating until well blended. Melt the chocolate carefully in the microwave and add to the butter mixture.
Sift the flour, baking powder, bicarbonate of soda and salt into the butter mixture. Pour the milk in and mix until a smooth batter forms. Stir in the currants.
Whisk the egg white until soft peaks form. Using a spatula, fold into the batter.
Fill the cupcake cases to two-thirds and bake for 25 minutes or until cooked through. Leave to cool completely.
Icing: Melt the sliced Bar-One chocolate and mix in the butter and milk. Sift in the icing sugar and beat until thick and smooth. Place in a piping bag and ice the cupcakes. Place pieces of Bar-One chocolate on top.