Preheat the oven to 350F and make the flax egg.
First, make the chocolate cookies. Add the vegan butter and powdered sugar to a bowl and cream together until fluffy. Then, add in the flax egg and vanilla and mix together.
Next, add in the all-purpose flour, sifted cocoa powder, and salt. Then, mix together into a thick chocolate cookie dough. If the dough is too sticky, add it to the fridge to chill and firm up for 10-15 minutes.
Roll the dough between 2 pieces of parchment paper until about ½ inches thick in height. Make sure not to roll the dough out too thin or it will be hard to cut out shapes.
Next, use a cookie cutter to cut out an even amount of top and bottom cookies (about 24 total). Each time you cut out cookies, form the scraps into a ball and roll out again until no dough is left.
Add the cookies to a lightly greased baking sheet or a baking sheet covered in parchment paper and bake for 10-11 minutes until the cookies are cooked through.
Remove the cookies from the oven and let cool on the baking tray for another 5 minutes. Then, move the cookies to a wire cooling rack to cool to room temperature.
While the cookies are cooling, make the vanilla buttercream filling.
Add the vegan butter to a large mixing bowl and whip until fluffy. Then, gradually add in the powdered sugar and whip together until you have a fluffy white frosting.
Next, add in the vanilla bean paste and whip it into the frosting for another 30 seconds until it mixed throughout.
Add the frosting to a piping bag and pipe onto ½ of the chocolate cookies. Then, add the remaining ½ of chocolate cookies on top to sandwich together. Serve & enjoy!