In a small saucepan or microwave-safe bowl, melt the butter. Once melted, remove from heat and transfer to a mixing bowl. Allow to cool for at least 5 minutes.
Preheat your oven to 350F and line a baking sheet with parchment paper.
While the butter is cooling, measure out your dry ingredients, peanut butter, and sugar.
Once the butter is cooled, add the peanut butter to the melted butter and stir until smooth. Add the brown sugar and whisk to combine.
Add the egg yolk and whisk together.
Add the dry ingredients and chocolate chips, and fold everything together. Your dough may look dry and crumbly; use clean hands to bring it together.
Divide the dough into 6 equal-sized cookies. Place additional chocolate chips or chunks on top of each cookie and gently press them down.
Place the cookie dough on the parchment-lined baking sheet and bake at 350F for 9-12 minutes.
Remove from the oven and use a round cutter or bowl to shape them if desired. Sprinkle with flaky sea salt and allow to cool on the sheet tray for a few minutes before transferring to a wire rack.