Line an 8in x 8in baking dish with parchment paper.
Brown the butter. Add butter to a small pot over medium-low heat. Cook until butter is amber in color and little brown flecks can be seen in the bottom of the pot.
Add peanut butter right away and reduce heat to low. Stir until peanut butter is melted. Add vanilla if using. Remove the pot from heat and stir in powdered sugar.
Stir until smooth and pour into prepared baking pan. Place in the fridge until set, about 2 hours.
Cut into small squares and top with sea salt (if using). Enjoy!