How to Make It
Make cookie bars: Preheat oven to 350°. In a large mixing bowl, whisk together all-purpose flour, almond flour, coconut flakes, baking soda, baking powder, and salt.
Using a mixer with the paddle attachment, cream butter and sugars on medium speed until light and fluffy, about 2 minutes. Add egg and beat well. Beat in lime juice, lime zest, and vanilla. Reduce speed to low and mix in flour in 3 additions.
Line a 9- by 13-in. baking pan with parchment paper so ends extend beyond length of pan. Trim to a 9-in. width (so it fits, widthwise, within pan without crinkling at the corners). Press cookie dough evenly into pan with the palm of your hand and smooth the surface. Bake until pale golden, about 20 minutes (it will be jiggly and puffed in center but will soon deflate and firm up). Cool completely in pan, about 2 hours.
Make matcha frosting: Sift powdered sugar and matcha into a medium bowl. In a separate bowl, whisk milk and vanilla together, then whisk into powdered-sugar matcha mixture. Spread frosting over cooled bars.
Chill bars until frosting is set, about 1 hour (or freeze about 30 minutes).
Make black icing: In a small bowl, whisk milk with powdered sugar. Add food coloring and mix well. Icing should be fairly stiff (if it's too fluid, it will smudge the cookies). Spoon icing into a small resealable plastic bag, squish into a corner, and cut off just the tip of corner to create a piping bag.
Using parchment overhang, lift cookie bar from pan onto a cutting board. Trim uneven edges, then cut into 2- by 2 1/2-in. rectangles with a sharp knife, wiping knife after every cut to keep it clean.
Using icing in piping bag, pipe Frankenstein hair, mouth, and stitches onto each cookie. Add candy eyeballs for the final touch.
*Find matcha green tea powder with the other teas at well-stocked grocery stores. Black food coloring and candy eyeballs are in the baking aisle.