In a mixing bowl combine the butter and sugars and beat until pale and fluffy. Add the eggs, one at a time, and beat until creamy. Mix in the vanilla.
In a separate bowl mix together the bicarb, flour and salt then gradually beat in to the butter mix. Finally stir in the chocolate chips or marshmallows.
Make tablespoon sized balls and arrange on a greased baking tray. Place in the freezer for 1 hour.
Place the chocolate bar pieces into a microwave safe bowl and microwave in 30 second increments until melted. Remove the dough balls from the freezer and using a cocktail stick or skewer to help you, roll the balls one at a time in the melted chocolate until completely covered. Transfer to a tray lined with baking parchment to set. Once the chocolate has set you can eat your yummy treats.
If you want to add sprinkles to the top of the balls, quickly sprinkle them on whilst the chocolate on the top of the cookie dough balls is setting. Be quick though because the chocolate sets in less than a minute!