Trim edges off all four sides of the brownie sheet to ensure they are straight, then cut the brownie just a little bit off center vertically. Cut the smaller of the two pieces in half, vertically, to make two strips. Divide each strip into thirds, horizontally, for a total of six big squares. Cut the remaining portion of the brownie in half horizontally. Divide one section in half again, horizontally, to make two strips. Cut each strip into fourths for a total of eight rectangles (you'll only need six). Divide the remaining section in half, vertically, and cut one of the pieces in half, vertically, into two strips. Divide each strip into thirds to create six small squares; save scraps for snacking. Place brownie pieces on a baking sheet and freeze for 2 hours.