Cut the butter into cubes and put them into a large saucepan. Place the pan over medium heat and stir until melted. Continue to stir the butter every ten seconds or so, until it bubbles and begins to foam. Once you see brown bits floating in the butter and a nutty aroma is released, immediately remove the pan from the heat. Pour the butter into a bowl to stop the cooking process, making sure to get all the browned bits from the bottom of the pan. Allow the butter to cool to room temperature, or set it in the fridge for around 30 minutes to speed up the process.

In the meantime, preheat an oven to 350 degrees F. Grease and line an 8x8 baking dish with parchment paper and set aside.

In a medium bowl, mix together the flour, oats, baking soda, and cinnamon until well combined. Set aside.

When the butter is cooled, pour it into a large bowl with the peanut butter and whisk until smooth. Add the brown sugar and granulated sugar and mix for 1-2 minutes. Beat in the egg and vanilla.

Reserve ⅓ cup of the candy and pretzels for topping. Once the wet ingredients are well mixed, add in the dry ingredients, chocolate, and the rest of the candy and pretzels. Stir the mixture together until a dough forms, then press the batter into the prepared pan (if it's too sticky, add 1-2 tablespoons more flour). Press the top of the dough with the reserved chocolates and pretzels.

Bake the cookie bars for 30-35 minutes, until golden brown on the edges and set in the middle. Once out of the oven, top the bars with flaky salt if desired. Let the cookie bars cool completely before removing them from the pan and slicing.