Preheat the oven to 200C/400F/Gas 6.
For the cookies, place the butter, sugar, water and egg into a large bowl and mix together well.
Sift the flour, baking powder, bicarbonate of soda and salt into the bowl and mix well to form a dough.
Fold in the chocolate chips.
Roll out the cookie dough onto a clean, floured work surface and, using a cookie cutter, cut into 10cm/4in circles.
Decorate each cookie with sugar-coated chocolates and place onto a greased baking tray.
Transfer to the oven to bake for 9-10 minutes, or until cooked through.
Meanwhile, for the chocolate sauce, place the butter, sugar, cream, cocoa powder, water and chocolate into a pan over a low heat and stir until combined. Cook gently for 4-5 minutes until thick and glossy.
To serve, place one cookie onto each plate. Add a large spoonful of ice cream and sandwich with a second cookie on top. Drizzle with chocolate sauce and serve.