In a stand mixer, add vegan butter and peanut butter. Beat on medium-high speed for 30 seconds. Add a half cup of granulated sugar and all of the brown sugar and beat until combined, scraping sides of bowl occasionally. Beat in the chia seeds and vanilla until combined.
In a separate bowl, combine the flour, baking soda, baking powder, and salt. Stir to combine. Pour the flour mixture into the peanut butter mixture and use a spatula to stir until the ingredients are combined. Add plant-based milk, one tablespoon at a time, and stir to achieve a soft dough. Cover and refrigerate for 30 minutes.
When the cookie dough is chilled, heat the oven to 375°F/190°C. Cover two cookie sheets with parchment paper.
Shape dough into 1-inch balls. Roll balls in reserved, granulated sugar to coat. Place rolled balls 1 to 2 inches apart on prepared cookie sheets. Using a fork to create crisscross marks on the top of each dough ball, slightly flattening each one.
Bake for 8 to 10 minutes, until light brown. Allow cooling for a moment on the cookie sheet before transferring to a wire rack to cool. Makes about 36 cookies.