Step 1
Preheat the oven to 300 degrees F (150 degrees C). Grease and line two 6-inch cake tins with parchment paper.
Step 2
Beat 1 cup vegan margarine using an electric mixer on low speed in a bowl until soft; add 2 cups powdered sugar and 2 tablespoons vanilla paste gradually on medium-low speed until creamy.
Step 3
Sift flour cornstarch baking powder baking soda and salt together in another bowl; mix well.
Step 4
Pour 2 1/2 cups soy milk into another bowl and add vinegar; stir until it thickens. Add vegetable oil and stir to combine.
Step 5
Alternate adding small amounts of flour mixture and soy milk mixture to sugar mixture beating on low speed until you have combined all ingredients. Divide batter evenly between the two prepared cake tins being careful not to fill over 3/4-full.
Step 6
Bake in the preheated oven until a toothpick inserted into the centers comes out clean 80 to 90 minutes.
Step 7
Remove from the oven and let cakes cool completely about 30 minutes.
Step 8
Mix shortening and vegan butter for frosting together in the bowl of a stand mixer using a paddle attachment on medium-low speed until fluffy. Add vanilla paste and mix to combine. Add powdered sugar and soy milk gradually until desired consistency is reached. Add cookie crumbs slowly and mix until combined. Fill a piping bag with frosting.
Step 9
Place 1 layer of cooled cake on a plate and frost top and sides with frosting; place remaining cake layer on top and frost completely.