Preheat the oven to 350°F and line two cookie sheets with parchment paper or silicone mats.
In a large bowl, using a handheld mixer or a stand mixer with the paddle attachment, beat the softened vegan butter and brown sugar for 1 to 2 minutes until creamy. If you are using brown butter here, let the brown butter return to room temperature before mixing with the brown sugar.
Add the vanilla or vanilla paste and the plant-based milk and mix to combine. Note if your plant-based milk is not at room temperature when you mix it, it might look like your mixture has broken, this is just the butter re-solidifying against the cold plant-based milk. Do not worry, this is normal and just keep going. Your cookies will still turn out!
Next, stop the mixer and add the flour, baking soda, and salt. Turn the mixer on low speed, and mix until just combined. If the dough seems a little dry, add an additional tablespoon or two of plant-based milk and mix to incorporate it.
Add in the chocolate chips and pistachios to incorporate, either by hand or using the mixer.
Optional: Cover the dough and place in the fridge for at least 30 minutes but up to 24 hours. This helps the cookie's texture and richness.
Using a 1.5 Tablespoon cookie scoop, scoop a cookie, then place the dough on the prepared baking sheets. Make sure to leave a little room between each cookie as they will spread while baking.* Then slightly flatten each dough ball so the cookies will bake evenly. Or after the cookies have been in the oven for 5 minutes, take them out and gently bang the cookie sheet on the counter to flatten a little.
Optional: Sprinkle each cookie ball with flaky sea salt.
Bake for 11 to 13 minutes, until the cookies are barely golden around the edges. They will be puffy and lightly colored. Do not overbake!
Let the cookies cool on the cookie sheet for 3 to 5 minutes, then transfer to a cooling rack for an additional 10 minutes. The cookies will firm up as they cool.
Store in an airtight container at room temperature for 5 to 7 days, or freeze for later.