Cover 2 -15 x 10 jelly roll pans with parchment papers.
In a mixing bowl, beat together cake mix, eggs, pudding mix and milk. Beat until smooth and combined. Evenly distribute cake batter between the two jelly roll pans. Spread evenly.
Bake both 12-15 minutes or until toothpick inserted comes out clean.
Let cake cool completely.
FOR FILLING:
In a mixing bowl, beat butter until fluffy. Add in powdered sugar, flour and vanilla. Add in milk and beat until fluffy.
Spread filling on just one of the cooled cakes. Take the second cake and place on top.
Sprinkle top with desired amount of powdered sugar. Cut into squares for serving