Whisk together eggs and brown sugar. Add in peanut butter and vanilla extract, mix until smooth.
In a separate bowl, mix together oat flour, baking soda and baking powder. Add dry ingredients to wet and mix until dough-like. Refrigerate for 20-30 minutes, or until you can roll dough into balls.
Preheat oven to 350 degrees F and line a baking sheet with parchment paper
While dough is in the refrigerator, add the eyes onto the peanut butter cups. To do this, add just a tiny bit of frosting on the back of each eye, then gently stick onto the peanut butter cup. Once finished, place them in the refrigerator (this is so the frosting can harden and the peanut butter cups are less likely to melt when you add them to the cookies)
Roll dough into 20 balls (~1 heaping tbsp each). Coat in sugar. Bake for 10-12 minutes.
Let cool for about 5 minutes, then gently add the peanut butter cups to the middle of each cookie. *Do not add them right away or they will melt and you'll lose the cute spider look of the cookies.
Once the cookies are fully cooled, add the frosting spider legs. Fill a piping bag (or a ziplock bag) with frosting, cut the very tip, then add 4 lines to each side of the cookie to create the legs.