Heat your oven to 375°F/190°C. Spray a 9-inch square baking pan with vegetable spray. In a large bowl, mix together the vegan butter, peanut butter, and sugars with a mixer until fluffy. Add vanilla, ground flax seeds, cornstarch, water, and 3 tablespoons of plant-based milk. Stir to combine.
Push the wet ingredients to one side of the bowl. On the other side, add the flour, baking soda, baking powder, and salt. Stir the dry ingredients together. Then incorporate the wet ingredients. If the batter is too crumbly, add additional plant-based milk until you achieve a cookie-dough-like consistency.
Stir in the chocolate chips. Pour batter into prepared baking pan. Sprinkle the top with sea salt.
Bake for 20 to 25 minutes, until the crust is lightly golden on top.
When done, remove from the oven and allow to cool before slicing.
Make the Vegan Caramel Sauce by adding brown sugar and plant-based milk in a saucepan over medium-high heat. Bring to a boil while stirring. Then reduce heat to medium and stir occasionally until the desired thickness is achieved, usually around 5 minutes. Remove from heat, add the tablespoon of vegan butter, stir to incorporate.