Preheat the oven to 350℉ (177℃). Melt the butter in 30 second increments until just melted but not hot.
Equip the paddle attachment to the stand mixer or use a hand held mixer. In the bowl, add the sugars and melted butter. Mix on medium speed until incorporated.
Add the egg and vanilla and mix on medium speed until combined. Add the dry ingredients and mix on medium speed until smooth.
Mix in chocolate chips and sprinkles with a rubber spatula. Scoop cookie dough into the prepared pan and gently press flat.
Bake on the center rack for 20-25 minutes or until just golden brown. The center will look slightly underbaked. Once it's done, move to a wire cooling rack. Let the cookie cool completely before decorating. (If edges are browning quickly, cover with tinfoil)
Vanilla Buttercream
Add the softened butter in the bowl of a stand mixer with a paddle attachment or use a hand mixer. Beat on high speed for one minute.
Turn the mixer off, add 1 cup of powdered sugar and mix on low for 30 seconds. Continue to add powdered sugar one cup at a time to avoid sugar getting everywhere.
Scrape down the bowl with a spatula. Add the salt, vanilla, and 1 tbsp of whipping cream and mix everything on medium speed for two minutes.
Change the speed to low and mix for 5 minutes. After 5 minutes, if the buttercream is too stiff and not smooth, add 1 tbsp of additional whipping cream and mix on low.
Decorating
Prepare a piping bag by adding the Wilton 1A tip into it and cut the tip with scissors. Fill halfway with buttercream.
Hold the piping bag vertically above the mini cookie cake. Slightly squeeze the bag and pipe a dollop, then pipe another slightly smaller dollop, and an even smaller third dollop. After piping the last dollop, pull the bag up and quickly away. Finish piping the buttercream ghosts all the way around.
Add on the edible candy eyes. Then sprinkle with Halloween nonpareils or your sprinkle of choice.
Store in an airtight container for up to 5 days. You can store the cookie cake in the refrigerator (makes the texture even chewier).