Preheat oven to 350 degrees Fahrenheit. Grease the bottom and sides of a 9- x 13-inch pan, press in a piece of Gefen Parchment Paper to cover the bottom and go up the sides. Grease the top and sides of the parchment. Set aside.

Place the sugar, eggs, oil and vanilla sugar into a large bowl and beat with an electric mixer on medium speed until combined. Add the ground almonds, ground pistachios and potato starch and mix well. 

Melt the chocolate either over a double boiler or in the microwave oven for one minute and stir, heat 45 seconds and stir, and then in 15 second increments until melted. Pour the chocolate on top of the nut bottom and spread. You can use the tines of a fork to make a design in the chocolate if you like. Place in the fridge until firm, about 30 minutes. 

Dump into the prepared pan and use the spatula to spread the dough back and forth to get it into the corners and evenly in the pan. Be patient; the spreading takes time. Bake for 30 minutes or until the stop is set and the edges just start to color; the edges should not be brown. Let cool 45 minutes.

Lift the edges of the parchment to lift the bar out of the pan and then slice into rectangular or square bars. Store in an airtight container or freezer bag as room temperature for up to five days or freeze.