Preheat your oven to 350°F (175°C). Line two large baking sheets with parchment paper or silicone baking mats so the cookies don’t stick.
In a large mixing bowl, cream together the softened butter and granulated sugar until the mixture is light and fluffy. This usually takes about 2 to 3 minutes of mixing.
Add the egg and vanilla extract to the butter mixture. Beat well until everything is fully combined and smooth.
In a separate bowl, whisk together the flour, baking powder, and salt. Gradually add the dry ingredients to the wet ingredients, mixing on low speed until a soft dough forms.
Roll the dough out to about 1/4 inch thickness. Cut the dough into rectangles (about 2×3 inches) to resemble books. Place them on the prepared baking sheets and bake for 8 to 10 minutes, or until the edges are just barely golden. Let them cool completely on a wire rack.
Once the cookies are cool, spread a thin layer of black frosting over each rectangle. Place one candy eyeball on the right side of the cookie. Use the silver or grey icing to draw “stitches” along the left side and around the eye to mimic the look of the movie’s spellbook.