Make the tart by placing the all purpose flour, powdered sugar, and salt in to a food processor. Pulse a few times until well combined. Add the cold vegan butter in tablespoon sized pieces and pulse until crumbly. Next, add in the non-dairy milk tablespoon by tablespoon and pulse until the dough gathers itself into a ball and cleans the sides of the food processor. Gather the dough in to a ball and wrap in cling wrap. Refrigerate for 1 hour.
After 1 hour, place the dough in the middle of a large piece of parchment and lightly flour a rolling pin. Roll out the dough until it's about ⅛in thick.
Gently press the rolled dough in to the tart pan and trim away the excess by rolling the rolling pin over the edges. Place the tart pan in to the freezer until the filling is ready. Preheat oven to 350F.
Melt the vegan butter and coconut oil over low heat and set aside to cool off.
Beat the aquafaba and cream of tartar in a stand mixer or with a hand mixer until firm peaks form. Turn down mixer to low and slowly add in the all purpose flour, organic cane sugar, organic brown sugar, and salt. Add in the cooled butter-oil mixture until just combined. You should be left with a mixture that resembles caramel.
Next, gently fold in the chocolate chips and chopped walnuts with a spatula.
Pour cookie mixture in to the chilled tart crust and bake for 1 hour. Let cool before serving.
Remove tart from pan and serve with Vegan Vanilla Ice Cream or Coconut Whip Cream! Enjoy!