For the batter, preheat the oven to 180°C. Line a 12-inch deep dish pizza pan or 10-inch cast-iron skillet with aluminum foil and spray with nonstick cooking spray.
Whisk the flour, oatmeal, baking soda, salt and cinnamon together in a medium bowl and set aside. Beat the butter, brown sugar and granulated sugar with an electric mixer on medium-high speed in a large bowl until well combined, about 2 minutes. Add the vanilla and eggs and beat until light in color, 2 to 3 minutes. Add the flour mixture and mix on low speed until well incorporated. Fold in the chocolate chips by hand using a rubber spatula.
Press the dough into the prepared pan and arrange 12 to 15 peanut butter cups on the top, pressing flush into the dough. Bake until deep golden brown and set, 20 to 25 minutes for the pizza pan or 30 to 35 minutes for the skillet. Cool in the pan 10 minutes. If removing from the pan, invert the cookie onto a large plate and remove foil, then upright onto a cutting board.
For the frosting: Beat the butter with the confectioners' sugar, milk and vanilla extract until firm enough to pipe. Transfer to a piping bag fitted with a round or star tip. When the cookie has cooled, drizzle with melted chocolate if using, then decorate as desired with the frosting and finish with sprinkles.