In a small bowl, mix together the soy milk and lemon. Set in fridge for an hour.
Meanwhile make the flax egg by combining 1 tablespoon of ground flax with two tablespoons of water. Refrigerate for 30 minutes.
Preheat oven to 350 F and lightly grease your whoopie pie pan.
In a bowl, whisk together the flour, powdered peanut butter, baking soda, powder, and salt.
In another bowl, cream together the butter with brown sugar. Add in the flax egg, and vanilla.
Mix the wet ingredients with the dry and slowly add in the vegan buttermilk.
Add about 2 tbsp of batter to each whoopie pie mold and bake for 10 minutes. Allow to fully cool while you continue with another batch.
Meanwhile, beat together the cream cheese, powdered peanut butter and sugar, to taste. Transfer to a piping bag and pipe onto the flat side of half of the whoopie pie cakes. Add some chia jam into the centre and sandwich with another whoopie pie cake. Enjoy!