Stir together the flour, sugar, baking powder, baking soda, salt and sugar in a medium-size bowl.
Pour in the melted butter, milk and vanilla. Stir everything together until you get a thick, sticky dough. Leave the dough to chill in the fridge for 10 minutes.
Meanwhile, make the cinnamon sugar coating. Combine the sugar and cinnamon in a small bowl and set to one side. Preheat the oven to 330F/165C 15 minutes before you're ready to bake and line a baking tray with parchment paper.
After 30 minutes of chilling, scoop out 1 tablespoon-size portions of the cookie dough and shape them into balls. (You should get 5 full-size balls and 1 ball just a little smaller.)
Roll each ball into the cinnamon sugar mix, place on the lined baking tray and bake in the preheated oven for 10 minutes, until the cookies have puffed up and turned light brown.
Remove from the oven and leave to cool on the baking tray to room temperature before eating.