Python Defined Function that Uses Previous Value Created

I have a dataset with two columns. One column is dates and the second is a multiple metric. I want to create a third column (New_Col) that looks at the current multiple and the previously created value and multiply them. So the first one would

Date           Multiple   
01-01-2020        1          
01-01-2020        1         
01-01-2020        3          
01-01-2020        1          
01-01-2020        1          
01-01-2020        1          
01-01-2020        4          
01-01-2020        1  

Function:

def new_func(df):

Output

Date           Multiple   New_Col
01-01-2020        1          1
01-01-2020        1          1
01-01-2020        3          3
01-01-2020        1          3
01-01-2020        1          3
01-01-2020        1          3
01-01-2020        4          12
01-01-2020        1          12