Heat oven to 350°. Prepare a 12-cup muffin pan by spraying with cooking spray. Set aside.
In a large mixing bowl, add brown sugar, baking soda, salt, vanilla, and egg. Stir with a whisk to combine together. Add the buttermilk, and whisk together until combined.
Start with 2 cups of flour and add it into the wet mixture. Stir together with a wooden spoon or spatula. Add more flour, in ¼ cup increments, until the dough forms a "ball" on it's own and it's not sticking to the sides of the bowl. I end up using about 2 ⅓ cups of flour when I make these. (see step-by-step pics above)
Lightly spray a countertop (or very large cutting board) with cooking spray and sprinkle some additional flour over top (about ¼ cup). Dump the dough onto the counter and knead it for 1-2 minutes. If needed add more flour if the dough is way too sticky and unmanageable.
Using a rolling pin, roll out the dough into a large rectangle. About the size of a cookie sheet 12"x18". Spread softened butter over the dough (this is easiest to do with your hands). In small bowl, mix together the brown sugar and cinnamon and then sprinkle evenly over the butter.
Starting at the long end, roll it up into a tight "log". Cut the log in half, and then cut each half into 6 equal slices. Place one cinnamon roll slice into each muffin cup.
Bake for 13-15 minutes. You do not want the muffins to be overly browned. They should really not have much color to them at all.
Stir together the powdered sugar and heavy cream to make the glaze, and then drizzle over the warm cinnamon roll muffins.