To make the dough, first warm the milk, this is a really important step! Using milk instead of water will yield a richer dough but you must get it warm enough to make the yeast come to the party but not too hot as the yeast hates that too. Microwave the milk in 30 second increments stirring in between each time until the milk is warm. 110°F or 45°C use a thermometer if you have one!
In the bowl of your mixer or a regular bowl if doing by hand, add the milk, yeast and sugar. Wait 5 minutes to make sure your yeast is good. You will know its good when it bubbles up and is foamy.
Now add in the egg and egg yolk, the butter and salt.
Add in the flour, one cup at a time. If you are using a mixer then keep it on the low setting with the dough hook for the entire process.
Once the dough has formed a ball, let it knead for 8-10 minutes in the machine or do it by hand on the counter. Do this until the dough passes the window pane test. See above.
Place dough in an oiled boil and cover with a clean kitchen towel or saran wrap. Set in a draft free location and let it rise until it has doubled in size. At room temperature this typically take 1 hour.
Once risen enough, Punch the dough down.
Place onto the counter and roll into a large rectangle about 20" x 14"
Mix the Filling
Combine the cinnamon, butter and brown sugar in a bowl. Note: If you like a really gooey cinnamon bun DOUBLE the filling.
Assemble the Buns
using a rubber spatula apply the filling evenly across the entire dough surface leaving a 1-2" boarder around the long edges.
Roll into a log. Pinch the long edge to the roll to close it.
Cut into 12 equal sized pieces
Place in a greased 9x13" baking dish.
Cover with a clean dish towel and let rise again until doubled in size (about 30 minutes)
Bake
In a preheated oven set to 375°F Bake the rolls for 25-30 minutes or until a bun in the center of the dish reaches 190°F