I'm sure it's better when made by a professional, but if you really want to try it, this site has a recipe: https://gluttodigest.com/cube-croissants-square-croissants-with-an-extra-dimension-of-texture/
It also mentions some places that sell it, but I'm guessing those are expensive and/or distant.
Buy puff pastry and a cube form.
Cut the dough into long triangles and roll into a spiral, fitting the triangle pieces into the box form diagonally.
Bake.
EZ
The Republic of France should just buy a cubesat launch slot and put this into orbit.
There’s gotta be a way to combine cube and croissant but I’m a bit too tired to figure it out. Cubessant? Croib?