Pre-heat the oven to 400F and line a baking sheet with puff pastry.
On a piece of parchment paper, roll out the defrosted puff pastry sheet into about 8x12 inch rectangle.
Brush the rolled out puff pastry with melted butter.
Mix together the brown sugar and cinnamon until combined. Then, sprinkle over the entire puff pastry, using hands or back of spoon to press it gently into the dough.
Start from the long edge side and tightly / gently roll up into a log, trying to tuck in any filling that’s spilling out as you go. Use any leftover butter to press and seal the seam so the log doesn't fall apart.
Cut into 1 1/2 inch slices using a serrated knife. Place spiral-side up on the parchment lined baking sheet and brush with egg wash if using.
Bake 22-27 minutes until puffy, golden brown and bubbling.
Allow to cool for a few minutes on the baking sheet and then move to a baking rack or plate.
Make the icing by mixing together the icing sugar, melted butter, 1 tbsp milk, vanilla extract and pinch of salt. If you'd like a bit looser icing, add another 1/2 tbsp milk.
Drizzle the icing over the cinnamon rolls and enjoy! Store in an airtight container at room temp for up to 2 days.