First add the butter and ¾ cup of the sugar to a mixing bowl and cream together until light and fluffy, about 2 minutes. Then, add in the flax eggs and vanilla extract and whip together about 10 -20more seconds to combine.
Next, add the sifted flour, baking powder, and salt to the mixing bowl. Mix until a thick, yet slightly sticky, cookie batter forms. Do not overmix or else you will form too much gluten and the cookies will not spread as much and will be tough.
Divide the dough into three equal portions. Then, add the matcha powder to one bowl, the sweet potato powder to the second bowl, and the charcoal powder to a third bowl. Cover all three bowls with plastic wrap and add to the fridge to chill for 1 hour.
Cover a baking sheet in parchment paper and preheat the oven to 350F. Next, remove the bowls of dough from the fridge. Scoop the cookie dough into 1 Tbsp balls (around 20 grams per dough ball). Then take 1 Tbsp of each flavor dough (matcha, sweet potato, and charcoal), press together, and gently roll into a ball between your palms. Then, roll in the bowl of the remaining ¼ cup sugar that was set aside until coated.
Place the cookie dough balls onto the parchment lined baking sheet. Then, slightly press each dough ball down with your palm. Make sure that each cookie is kept about 2 inches apart since they will spread slightly while baking. Then, press a few eyeball sprinkles into each cookie or wait until after baking to add any sprinkles as they may melt while baking.
Bake for 11-13 minutes until bottoms are lightly browned and the cookie edges are fully cooked with soft centers. The cookies will continue baking after they are removed from the oven so the centers should be soft and just cooked, but not glossy or raw, when they are removed from the oven. Please be careful not to overcook these cookies as they will lose the chewy texture if overcooked.
Remove the cookie tray from the oven and immediately tap the tray on the counter to produce a more “wrinkly” cookie. Then, while the cookies are still warm, press in any remaining googly eye ball sprinkles so they stick. At this point you can also swirl the warm cookies between a round pastry/cookie cutter to make them round.
Let the cookies cool on the baking sheet for 5 minutes. The cookies will be soft and puffy out of the oven and will naturally deflate a little while cooling. After 5 minutes, move to a wire cooling rack to finish cooling. Enjoy!