To make the dough. In the bowl of a stand mixer, combine the eggnog, yeast, honey, eggs, butter, 3 ½ cups flour, and salt. Using the dough hook, mix until the flour is completely incorporated, about 4-5 minutes. If the dough seems sticky, add the remaining ½ cup of flour. It's better to have a sticker dough rather than a dry do, so add flour 1 tablespoon at a time.Cover the bowl with plastic wrap and let sit at room temperature for 1 hour or until the dough has doubled in size.Meanwhile, make the filling. In a small bowl, combine the brown sugar, and cinnamon. Line a 9x13 inch baking dish with parchment. Lightly dust your work surface with flour. Turn out the dough, punch it down, and roll it into a rectangle about 10 x 16 inches. Spread the softened butter evenly over the dough. Sprinkle the cinnamon sugar evenly over the butter and lightly push into the butter. Starting with the long edge closest to you, carefully roll the dough into a log, keeping it fairly tight as you go. When you reach the edge, pinch along the edge to seal. Using a sharp knife, cut into 12-15 rolls. Place the rolls in the prepared baking dish. Cover with plastic wrap and let rise 20-30 minutes or transfer to the fridge to rise overnight (this is ideal if you have time).Remove the rolls from the fridge. Preheat the oven to 350 degrees F. Bake the rolls for 25 to 30 minutes, or until golden brown.Meanwhile, make the frosting. In a small saucepan, melt the butter over medium heat. Cook for about 5 minutes, until the butter is lightly browned and smells toasted. Immediately remove from the heat. Let cool.In a medium bowl, beat together the cooled browned butter, cream cheese, confectioners' sugar, and vanilla until smooth. If the frosting is too thick for your liking, add a tablespoon of eggnog or milk.Spread the frosting over the warm rolls. Sprinkle with flaky sea salt (if desired) and serve.