Add the ½ cup vegan butter to a medium saucepan or frying pan and heat over medium-low heat. Once melted, add 1 tablespoon aquafaba (if needed). Cook, stirring frequently, until butter turns golden brown and develops a nutty aroma, about 15-20 minutes. Pour browned butter into a heat-resistant container and refrigerate until resolidified, about 1 hour.
Preheat oven to 350°F. Line a baking sheet with a Silpat mat or parchment paper.
In a medium mixing bowl, whisk together 150g all-purpose flour, ½ teaspoon baking soda, and ½ teaspoon baking powder. Set aside.
In a separate mixing bowl, cream the resolidified browned butter with ½ cup evaporated cane sugar and ¼ cup brown sugar using an electric mixer until light and fluffy, about 2-3 minutes.
Add ½ cup tahini, ½ teaspoon kosher salt, and 1 teaspoon vanilla extract to the butter mixture. Mix until well combined.
Add the prepared flax egg and mix until incorporated.
Add the flour mixture to the wet ingredients and mix just until a dough forms.
Fold in 60g dark chocolate chunks until evenly distributed throughout the dough.
Divide dough into portions and roll into balls (about 2 tablespoons each). Let the dough balls rest for 30 minutes in the fridge.
Place rested dough balls on the prepared baking sheet, spacing them about 2 inches apart.
Bake for 15 minutes, or until edges are lightly golden. Allow cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack.
Sprinkle the cookies with flaky sea salt while still warm if desired.