Preheat the oven to 180C | 350F. Line a small deep-sided tin (28x17cm/11x7in) with baking paper, letting the edges overhang.
Cream the butter and sugar in a stand mixer fitted with a paddle attachment.
Add the egg and beat to combine. Stop the mixer and scrape the bottom and sides of the mixing bowl.
Add the flour, salt, baking powder, bicarbonate of soda and cinnamon and beat to combine. The dough should form a ball around the blade.
Press the dough into the prepared tin and then press chocolate buttons over the top in straight lines - bearing in mind that you will cut the cookie into slices.
Bake for 18-25 minutes until the edges start to colour. Cool slightly then remove from the tin and carefully slice with a sharp knife. Spread the slices on a baking tray and return to the (turned off but warm) oven to dry out slightly.
Prepare the sauce. Gently heat the milk and vanilla until small bubbles appear around the edge of the saucepan then pour over the chopped chocolate. Let it stand for a couple of minutes then stir so the chocolate melts.
Return to the pan and add the marshmallows – stir over low heat until the marshmallows melt. (You can also heat in the microwave on medium setting in short bursts.)
Serve the cookies with little pots of the sauce so that everyone can dip to their hearts content!
To make the sauce in the Vitamix, add the chocolate, vanilla and milk in the container then run the soup programme. Add the marshmallows and use the variable speed to mix until they are blended into the sauce. The sauce will get set as it cools.