In a medium-sized mixing bowl, combine the dry ingredients (flour, baking soda, and salt). Set aside.
In a large bowl, using a hand mixer or stand mixer, cream the butter, peanut butter, and sugars for 2 minutes. Scrape off the sides of the bowl with a spatula.
Add in the egg, and vanilla extract, and mix again until well combined.
Add the dry ingredients to the wet ingredients and mix until just combined (do not overmix the dough). Cover with plastic wrap, and chill in the fridge for at least 30 minutes and up to overnight.
Line two baking sheets with parchment paper. Divide the cookie dough into 1.5 tablespoon-sized balls using a medium-sized cookie scoop.
Gently roll each ball in granulated sugar to coat from all sides, and arrange on the baking sheets 2 inches apart.
Bake for 8-10 minutes. The cookies will puff up and have cracked tops. Remove from the oven and cool for 5 minutes only. No longer though, letting them cool for too long will make the next step difficult.
Unwrap and press a Franken cup upside down in the center of each cookie, then remove onto a cooling rack. Allow the cookies to cool completely before decorating, I like to pop them in the fridge to speed up the process (20-30 minutes is enough).
Push 2 candy eyes into each Franken cup (they will stick fine, the Franken cups will still be a little soft). Draw the face, and sprinkle the hair with chocolate sprinkles.
Notes
Vanilla: I personally prefer a high-quality pure vanilla extract and think it makes a big difference. But if imitation vanilla is your thing, then go for it.
Storing. Store the baked cookies for 5-7 days in an airtight container at room temperature.
Freezing. You can freeze the dough before baking, and before rolling in sugar.