Heat the oven to 390°F (200°C) (355°F/180°C convection). Generously butter a 12-cup deep muffin pan.
First, prepare the filling. Melt the butter and leave in a warm place so that it remains liquid. Mix together the light brown sugar and cinnamon until no lumps remain, then set aside.
Now make the dough. In the bowl of a stand mixer with a paddle attachment, combine all the dry ingredients with the cubes of butter and mix until you have a coarse meal. Slowly pour in the cold milk while the mixer is running, until the dough forms into a ball and comes away from the bowl. Turn the dough out onto a lightly floured surface and leave to rest for a few minutes. Fold the dough gently over itself once or twice to pull it all together let the dough rest a second time, for 10 minutes.
Clear a large surface, dust lightly with more flour, and roll out the dough into a large rectangle until almost 1/8 inch (5mm) thick. Brush the surface of the dough with the melted butter and, before the butter hardens, sprinkle the cinnamon sugar on to the butter. You want a good, slightly thick layer.
Now roll the long side, keeping it neat and tidy. Gently tug the dough toward you to get a taut roll while rolling away from you in a spiral. Once it’s all rolled up, gently squeeze the roll to ensure it’s the same thickness throughout. Use a sharp knife to cut the roll crosswise into 12 even slices. Take a slice of the cinnamon roll, peel back about 2 inches (5cm) of the loose end of the pastry and fold back under the roll to loosely cover the bottom of the roll. Place in the muffin pan, flap side down. Repeat with remaining slices.
Bake the buns for 25 minutes, or until golden and well-risen. As soon as they're out of the oven, flip them over onto a wire cooling rack so that they don't stick to the pan. Dip each cinnamon bun into a bowl of sugar. These are best served right away while still warm, but can also be served at room temperature or re-warmed in a low oven (especially good with a scoop of ice cream for dessert).