Preheat oven to 350°F/180°C and line two baking trays with parchment paper. Set aside.
Melt the butter over low heat. Once melted, remove from heat and cool while you prep the rest of your ingredients.
1/4 cup butter
In a large bowl, whisk in the cooled melted butter, brown sugar, and white sugar for one minute until combined.
1/3 cup dark brown sugar, 1 tbsp white sugar
Add in egg yolk, vanilla, and peanut butter, and continue whisking.
1/3 cup peanut butter, 1 egg yolk, 1 tbsp vanilla
Add in flour, baking soda, and salt, and mix until a soft dough forms. Be careful not to over mix.
1/2 cup all-purpose flour, 1/4 tsp baking soda, 1/4 tsp salt
Scoop 4 cookies onto your prepared baking tray. Sprinkle extra sugar on top (optional, but recommended).
extra white sugar to sprinkle on cookies
Bake for 10-12 minutes, or until edges are slightly golden brown. Sprinkle flakey sea salt once removed from oven.
flakey sea salt
Cool the cookies, and enjoy!