Preheat the oven to 350°F. Line a 9×9-inch baking pan with parchment paper, and grease the edges. Make the plant-based butter milk by mixing together the plant-based milk and apple cider vinegar, then set aside to curdle for 5 to 10 minutes.
In a large bowl, whisk together the pumpkin puree, sugar, brown sugar, melted plant-based butter, maple syrup, vanilla, and your milk mixture. Add in the pumpkin spice, baking powder, baking soda, and sea salt and whisk together. Then add in flour, along with the plant-based buttermilk, and gently whisk the flour into the wet until there are no more dry clumps. Be careful not to over-mix the batter!
Pour the batter into the prepared pan and place the pumpkin bars in the oven to bake for 22 to 25 minutes, or until a toothpick inserted into the center comes out clean.
Once done, remove the bars from the oven, and allow the bars to cool in the pan for at least 10 minutes. Then carefully transfer them to a cooling rack to cool completely before making the cream cheese frosting.
Once the pumpkin bars are completely cool, you can begin to make the plant-based cream cheese frosting. Start by creaming together the plant-based cream cheese and plant-based butter in a large bowl, using either a hand mixer or a stand mixer. Add in the sifted powdered sugar in 1 cup increments, mixing as you go. Once the powdered sugar is fully incorporated, add in your vanilla. Then mix until light and fluffy, about 2 to 3 minutes. You can use a tablespoon of plant-based milk if necessary to help incorporate the powdered sugar.
Spoon the frosting onto the tops of the pumpkin bars. Spread it evenly until it reaches the edges of the bars. Slice and serve.
Storage Instructions: Store any leftover slices in an airtight container for up to 5 days in the fridge or up to 2 months in the freezer.