Chocolate Sandwich Cookies

1. For cookies, combine flax seed and water in a dish. Set aside. Cream margarine and sugar in the bowl of an electric mixer. Mix until pale and fluffy. Add flax mixture to margarine mixture and beat well. Combine cocoa powder and flour, and add to the margarine mixture with the mixer on low. Mix until just combined.
2. Divide dough in 1/2 and shape each 1/2 into discs. Wrap each disc in plastic or wax paper and refrigerate for about 1/2-1 hour.
3. For filling, beat margarine and shortening until fluffy. Add the sugar and salt and beat for a few minutes. Add the vanilla and milk. Beat for another 5 minutes. Adjust the amount of milk based on desired filling consistency.
4. Preheat oven to 350 degrees F, and line a baking sheet with parchment. Roll 1 disc on a surface lightly dusted with icing sugar or roll between 2 sheets parchment. Roll out dough to about 1/8" thick. Cut out cookies with a 1 1/2- 2" cookie cutter, and place about 1/2" apart on prepared baking sheet.
5. Repeat with scraps of dough. I used 1 disc for the tops and 1 disc for the bottoms. Bake cookies for about 10 minutes. Cool completely before assembling. For assembly, spread about 1 teaspoon filling on 1 cookie and place another cookie on top. Gently press the cookies together.
These cookies freeze well! You can also make them bigger, if you want. Just adjust the baking time a bit. I made a couple larger ones for the boy and he said they were a lot like oreo cakesters, but with a little more firmness to them (which in his opinion, made them better than cakesters). He was also shocked to find out that these were vegan!
The first time I made these, I used leftover frosting from making the cookies n' creme VCTOTW cupcakes, so feel free to add finely chopped pieces of your favorite sandwich cookie to the frosting!
Source of recipe: I fiddled around and came up with this cookie recipe. The filling is a modification of VCTOTW's buttercream frosting.