Combine butter and sugars in the bowl of a stand mixer and beat until well combined (about 1 minute).

Add peanut butter, stir well.

Add eggs and egg yolk, one at a time, pausing to scrape down the sides and bottom of the bowl after each addition.

Stir in vanilla extract.

In a separate, medium-sized bowl, whisk together flour, cornstarch, baking soda, and salt.

Gradually add flour mixture into the peanut butter batter, pausing occasionally to scrape down sides and bottom of bowl so that all ingredients are well-combined.

Place your dough in refrigerator and chill for 30 minutes to one hour (start with 30 minutes, if the dough is too sticky or cookies come out too flat, return for another half an hour).

Preheat oven to 375F (190C) and line cookie sheets with parchment paper. Pour remaining ½ cup of sugar into a small bowl and set aside.

Once dough has chilled, remove from refrigerator and roll into 1 ½-Tablespoon-sized balls. Roll through sugar and place on prepared cookie sheet at least 2" apart.

Bake on 375F (190C) for 8-10 minutes, remove from oven and allow to cool on cookie sheet for 5 minutes. Use your frozen Reese's (I keep mine wrapped) and gently make an indent by pressing the top of the Reese's into the top of each peanut butter cookie. Allow cookies to continue to cool completely on cookie sheet before proceeding.

Marshmallow Cobweb Instructions: Place about ¼ cup of your mini marshmallows in a small microwave-safe dish.

Microwave for about 10 seconds or until marshmallows begin to puff.

Remove and use a fork or spoon to stir, deflating your marshmallows.

Using your thumb and forefinger of each hand, pinch some of the marshmallow filling. Stretch and pull the marshmallow apart with your fingers and drape it over your cooled cookies (See picture above for a visual guide).

Marshmallow will cool quickly and become too difficult to use, once that happens with your first bowl of marshmallows, move on to a new bowl with another ¼ cup marshmallows and repeat steps until all cookies are decorated.

Allow marshmallow to set for several minutes before proceeding with the chocolate -- the longer you wait the less sticky it will be.

Spider Decorating Instructions: Prepare your dark chocolate melting wafers according to package instructions. Allow to cool 5 minutes before proceeding (if chocolate is too hot it will be harder to control when piping).

Pour into a small ziploc bag and snip a tiny, small corner off of the bag (the width that you want your spider legs to be).

Pipe a small amount of chocolate into the indent on your peanut butter cookie. Work with one cookie at a time as the chocolate will harden fairly quickly.

Place one Reese's cup (top side/larger surface facing down) into the melted chocolate.

Pipe four chocolate lines on either side of the Reese's for the spider "legs".

Dab a small amount of the chocolate on the back of a candy eye and press it against the side of the Reese's cup, repeat with another eye for your spider's eyes.