Use a heavy spatula or rolling pin to press the croissants flat.
Combine softened butter and brown sugar in a bowl. Spread the sweetened butter over the outside of the flattened croissants.
Heat a large nonstick skillet over medium. Lay a piece of parchment on top and weigh down with another pan or a bacon press for even contact. Cook 2–3 minutes until edges are deep golden and caramelized. Flip and cook 1–2 minutes more. Move to a rack or plate for 2 minutes to set the caramel shell (it crisps as it cools).
Dust with powdered sugar or your favorite sweet and savory toppings. Serve warm for peak shatter-crisp edges.