For each chocolate: Melt the chocolate and butter in a saucepan. Whisking to incorporate completely.
In a large mixing bowl add the powdered sugar and eggs. Using a hand mixer beat well, carefully as this dusts up quite a bit!
Next add the melted chocolate mixture and vanilla. Mix thoroughly to incorporate. This will be smooth and thick.
With a rubber spatula, pour dark chocolate fudge mixture into a greased 9x13” pan. Repeat with white chocolate fudge mixture and with rubber spatula make swirl patterns to incorporate both dark and white chocolate. Refrigerate to firm at least 3 hours. Slice into bite sized pieces and enjoy!