Descending order unique squares

[OmegaRed94]

Good day, Dear Pythonistas

I have the following problem: Given a list, we should write a function that returns sorted in descending order unique squares of numbers contained in this list

I have got the solution in the following way, it works correctly:

def squares_list(l):
    for elem in l:
        empty_list.extend(elem)
    new_list=list(set(empty_list))
    new_list.sort(reverse=True)
    squares=[i**2 for i in new_list]
    return squares
print(squares_list([[1, 2], [3], [4], [3, 1]]))

Could you, please, suggest, how I could write this code or the solution in the shortest possible optimal way (a smaller number of characters)?