Start my creaming together the sugar and 2 tablespoons of the butter until well blended. Then add the vanilla and stir to combine.
Add the flour and stir until you have a dry crumbly mixture. Tip: heat treated flour can be quite lumpy, so may benefit from sifting when adding to the dough.
Next add the milk, stirring until well mixed and smooth. The consistency should be spreadable, a bit wetter than the average cookie dough.
Place a piece of plastic wrap on a work surface and dollop the cookie dough out onto this. Then place another piece of plastic wrap on top, essentially sandwiching the dough between the plastic wrap.
Press the plastic wrap to spread the dough out (you can use your hands here and also a rolling pin) until you have a rectangle of the dough, measuring roughly 12" wide by 5" long. The dough should be thin, less than ¼" thick.
Place the dough rectangle (still sandwiched by the plastic wrap) in the refrigerator to chill and firm up for at least half an hour.
Meanwhile, prepare your cinnamon filling by mixing together the remaining 1 tablespoon of butter with the cinnamon and the brown sugar until you have a spreadable paste, similar to making normal cinnamon rolls.
After the dough has chilled, remove this from the refrigerator and uncover the top of the dough (leaving the plastic on the bottom).
Spread the cinnamon mixture over the top of the dough.
Gently, using the plastic wrap to help you, roll the dough up into a log (this should be rolled up along the 5" short side, so you're creating a 12" long log).
Once you've formed a tight log, wrap this in plastic wrap and place in the refrigerator to chill and firm up for about an hour.
After the log has firmed up, unwrap and use a sharp knife to slice into ¼" wide pieces. Place these on a parchment or plastic wrap lined tray and place in the freezer until needed.