You can use this method to grate the butter and combine it with the flours for the crust. Or you can follow the directions below and pulse the butter and the flours in a food processor. You don't need a fancy processor, I use a tiny $20 model and it works great for little tasks like this one.
If you leave the bars in the refrigerator overnight, let them sit at room temperature for at least 15-20 minutes before slicing. Otherwise they will be very hard to cut and the chocolate will likely crack.