Preheat oven to 375°F. Brush the wells of a standard muffin tin generously with melted butter, making sure the bottoms and sides are well coated.
In a small bowl, mix together brown sugar and 1 tbsp cinnamon. Set aside.Lay out each sheet of puff pastry on a lightly floured surface. Spread 3 tbsp softened butter evenly over each sheet.
Sprinkle the cinnamon–brown sugar mixture evenly over both sheets of puff pastry, pressing gently so it adheres. Starting from the long side, roll each sheet into a tight log.
Using a sharp knife, slice each log into 6 equal pieces, giving you 12 morning buns total.
Place each slice cut-side up into the buttered muffin tin.Bake for 22–28 minutes, or until the buns are puffed, deeply golden, and caramelized around the edges.
While the buns are baking, combine 1/2 cup granulated sugar with 1 tsp cinnamon in a shallow bowl.Once the buns come out of the oven, let them sit for 5 minutes, then remove from the muffin tin. Immediately roll or toss the warm buns in the cinnamon sugar mixture to create a crunchy, sparkling coating.
Enjoy warm! These buns are best the day they’re made but can be reheated at 300°F for 5–7 minutes.