In the bowl of an electric mixer, add the milk and sprinkle on the yeast. Let sit until the yeast starts to foam, 1-2 minutes. Once foamy, stir in the sugar, egg yolk, and melted butter. Stir in the flour and salt with a wooden spoon until everything comes together into a ball of dough.
Knead with the dough hook on medium for 8 minutes. The dough should be soft and slightly tacky. Alternately, knead by hand for 8-10 minutes on a floured surface.
Lightly oil a bowl and place the dough inside. Cover with plastic wrap and a kitchen towel and let rise for 1 to 1 and a half hours or until doubled in size.
On a lightly floured surface, tip out the dough and roll out to a large, long rectangle. Spread the butter evenly, all the way to the edges. Whisk together the cinnamon and sugar in small bowl then spread evenly onto the dough. Roll into a tight roll, cut into 12 even pieces. Place in a small 8 inch cast iron pan or small baking pan.
Cover and let rise at room temperature for 30 minutes to 1 hour, or until doubled. If making ahead, cover tightly and place in the fridge overnight.
Heat the oven to 350°F. Bake for 20-25 minutes, or until slightly golden and just cooked through or the internal temp comes to 190°F. Five minutes before the buns are almost done baking, melt the butter and sugar together to make a glaze. Brush the glaze on during the last five minutes of baking.
Let the buns cool for five minutes, then sprinkle with flaky sea salt and enjoy!!