Chewy Pistachio Cookies with Orange and Dark Chocolate Drizzle

0 Prep: Preheat the oven to 350F. Spray or line a large baking tray with parchment paper or a silicone mat and set aside. Prepare the flax “egg” by mixing the flaxseed and water well; set aside for at least 5 minutes to let thicken.
1 Chop the Pistachios: Add the pistachios to a food processor with an S blade attachment and pulse until chopped into small pieces. You can also opt to chop the pistachios by hand. Set 1/2 cup of pistachios aside to roll the cookies in later; use the remaining ~3/4 cup of them in step 5.
2 Cream the Butter: Add the cane sugar to a large bowl or stand mixer. If you are using the optional orange zest, add it to the sugar and massage the two together until evenly combined. Add the softened butter and brown sugar to the bowl as well. Use a stand mixer or electric mixer to cream the butter and sugar together on low speed for 3 to 5 minutes, until the butter is smooth, light, and fluffy. Scrape the sides of the bowl with a spatula as necessary.
3 Wet Ingredients: Add the flax “egg” and vanilla to the bowl and beat again, until the mixture is smooth and combined.
4 Dry Ingredients: Add the flour, cornstarch, baking soda, baking powder, and salt. Mix well until a thick, non-sticky dough forms. Add the ~3/4 cup of chopped pistachios to the batter and fold into the dough using a spatula.
5 Form the Cookies: Spread the reserved 1/2 cup of pistachios onto a small plate. Use a cookie scoop or spoon to roughly form 12 cookies with ~3 tablespoons of dough per cookie. Roll each cookie into a smooth ball, then roll the top half of each ball into the chopped pistachios. Place on the prepared baking sheet with the pistachios on top, but do not flatten the dough; it will spread as it bakes in the oven. Repeat with the remaining cookie dough.
6 Bake: Bake in the middle rack of the oven for 10 to 12 minutes; the cookies will look puffy and pale, but the bottom edges should be slightly golden.
7 Cool: Take the cookies out of the oven and place the baking sheet on a wire rack. Let the cookies cool on the tray for 10 minutes, then transfer to a different wire cooling rack and let cool completely.
8 Chocolate Drizzle (Optional): Melt the chocolate using a microwave or double boiler (see recipe notes for steps). Drizzle the melted chocolate over the cookies once they have been transferred off of the baking sheet onto a cooling rack. (Tip: If you have any extra pistachios from step 6 I like to sprinkle them on top of the chocolate drizzled for an extra pop of color). Let the chocolate fully harden before stacking or storing the cookies.
9 Serve & Store: Enjoy the cookies immediately, or store in an airtight container for up to 5 days. You can also store the baked cookies in the freezer for up to 2 months, but I would recommend storing the cookie dough balls instead and baking when desired :)