Combine buttermilk and butter in glass measuring cup. Microwave on high until butter melts and mixture is warm but not too hot to stick your finger in. Pour into the bowl of a stand mixer fitted with paddle attachment.
On medium speed, beat the sugar, egg, yeast and salt into the buttermilk mixture. Reduce speed to low and beat in the flour. When a sticky dough comes together, switch to the dough hook and knead for about 6 minutes on low speed. It will not form a ball, it will be very soft. Don't add more flour.
Cover bowl with a clean towel and let dough rise until fully doubled in volume, at least 2 hours.
Mix brown sugar and cinnamon in medium bowl.
Transfer dough to a well-floured work surface. Roll out to a rectangle (aim for a roughly 15" long edge. The closer you make it to a square, the more layers of swirl you'll have). Spread butter over dough, leaving 1/2-inch border. Sprinkle cinnamon sugar evenly over butter. Starting at 1 long side, roll dough into log, pinching gently to keep it rolled up. With seam side down, cut dough crosswise with thin sharp knife into 3/4" slices.
Line a 9x13" pan with parchment paper. Arrange the rolls in the pan, putting any smaller ones in the middle and bigger ones in the corners for even baking. Cover with the kitchen towel and let dough rise again until doubled in volume, about 45 minutes.
Position rack in center of oven and preheat to 375°F. Bake rolls until tops are golden, about 20 minutes. Don't overbake. Use a knife to pull apart the middle rolls to make sure it's not still sticky-doughy. The outer buns should register 190ºF while the middle ones remain lower. Remove from oven and cool in the pan for 10 minutes.
Combine cream cheese, powdered sugar, butter, and vanilla in medium bowl. Using electric mixer, beat until smooth. Spread glaze on rolls. Serve warm or at room temperature.