Preheat oven to 400˚F. Grease a 12-cup muffin tin with cooking spray or butter.
In a small bowl, stir together 2 Tbsp granulated sugar, brown sugar, lemon zest, and salt.
Sprinkle a clean, flat work surface with 1 Tbsp granulated sugar. Place one sheet of puff pastry on top.
Spread the puff pastry with 2 Tbsp softened butter. Sprinkle with half of the lemon sugar.
Use a rolling pin to lightly roll the puff pastry evenly. This will help the sugar stick on both sides!
Tightly roll up the puff pastry from its longest side.
Repeat steps 3 through 6 with the other sheet of puff pastry.
Cut each long into 6 even rounds.
Place each round into a muffin cup, cut side up.
Bake for 20 to 25 minutes until golden brown and cooked through. Carefully remove buns to a wire rack to cool slightly.
Add remaining granulated sugar to a small bowl. Toss each bun in the sugar to coat.
Serve warm.