Prior to starting the cookies, freeze 1 tbsp. balls of the peanut butter so that it's *much* easier to work with later on. I used a 1 tbsp. cookie scoop and scooped 8 balls of peanut butter onto a parchment lined sheet. Then, just pop the tray in the freezer until you're ready to assemble the cookies.
In a small saucepan set over medium heat, brown the butter. Stir the butter occasionally until it turns amber in color and develops little brown specks. At this point, weigh your brown butter to make sure you have at least 90g. If you have a little less, mix in some water until you reach 90g.
Transfer the brown butter to a medium sized mixing bowl and set it in the freezer for 5 minutes to chill. In the meantime, chop up your chocolate bar if using one.
Once the butter has cooled, whisk in both sugars.
Whisk in the egg and vanilla until well combined.
Add in the flour, oats, baking soda, baking powder, salt, and cinnamon. Use a rubber spatula to fold the mixture together until a few flour streaks remain.
Add in the chopped chocolate and fold just until no flour streaks remain.
Scoop out heaping 3 tbsp. scoops of the cookie dough. Use your palm to flatten the cookie dough ball down, then place a frozen ball of peanut butter in the center of the flattened cookie.
Fold the overhanging cookie dough on top of the peanut butter, making sure it is completely trapped inside each cookie
Place the filled cookie dough balls into the freezer for 15 min. Preheat the oven to 350°F / 175°C.
Evenly space out the cookie dough balls on a parchment lined cookie sheet. If desired, melt some extra peanut butter and use a spoon to top each cookie dough ball off with some of the extra peanut butter, as well as some extra chocolate.
Bake for 12-14 minutes, until the edges of the cookies turn golden brown.
Once the cookies come out of the oven, I recommend "scooting" them. This just means take a bowl, cup, or biscuit cutter that is slightly larger than the cookie, place it on top of the cookie, and swirl the bowl around to make the cookie a little more round.
Allow the cookies to rest on the hot baking sheet for at least 5 minutes before transferring to a wire rack to cool. Enjoy!