Preheat the oven to 350 degrees F. Spray 2 baking sheets with non-stick cooking spray or line with parchment or a silpat.
Sift together the flour, cocoa powder, baking powder, baking soda and salt. Cream the brown sugar, vegetable shortening and oil, add the egg and vanilla in slowly until smooth. On a low speed, add the flour mixture, until incorporated - the batter will be quite thick. Scrape the side of the mixing bowl then pour the buttermilk in slowly down the side of the mixing bowl. Increase speed to medium to incorporate.
For larger cookies divide the batter into 18 - 20 equal portions. ( The batter is extremely sticky and elastic, I recommend using an ice cream or portion control scoop with a release grip). Place 9 or 10 scoops of batter per sheet. Smaller scoops may be used for smaller cookies. Wet your fingers and flatten the mounds of dough ever so slightly, to help the batter spread while baking.
Bake the larger size for about 15 to 18 minutes, the smaller size for about 11 minutes. They should be set but still somewhat soft when pressed with a finger. Allow to cool on a rack before filling.
Scoop the ice cream and sandwich between 2 cookies just prior to serving. Alternately, fill them in advance, wrap in plastic and freeze until ready to serve.
Makes about 9 or 10 ice cream sandwiches.