In the bowl of the stand mixer, cream together the sugar, butter, and eggs. Add the chocolate syrup and the vanilla and mix well.
Stir the flour and salt together in a small bowl, then add to the mixer bowl a little at a time. Mix well.
Spread the mixture evenly in the prepared jelly roll pan. Bake for 20 minutes, until the middle is just set. Remove from the oven and allow to cool completely, about 30 to 60 minutes.
Creme de Menthe Layer
In the bowl of the stand mixer, mix together the powdered sugar, butter, and Creme de Menthe. Spread all over the chocolate bottom layer, and then put the pan in the refrigerator.
In a microwave safe bowl, melt the chocolate chips and butter together and stir until combined. Allow to cool slightly, and then spread all over the top of the mint layer. Put the pan back in the refrigerator and allow to chill thoroughly. Cut into small squares and keep in an airtight container in the refrigerator.
You can use mint extract rather than Creme de Menthe, but you won't cut out much--if any--alcohol since most true mint extracts have an alcohol base with as much as 90% alcohol.