Set the butter in a heatproof bowl, then in the microwave and warm it for about 1 minute, until the butter is almost but not fully melted.
Add the chocolate chips, stir slightly then return it to the microwave for another 30 seconds. Stir the mix until the chocolate is all melted and it’s smooth.
Add the sugar and whisk very well, for a full minute. Add the sea salt, baking soda, vanilla and peanut butter then whisk very well for another minute.
Crack in the egg and yolk, whisk well. Add the flour (if it’s lumpy shake it through a fine mesh sieve) and stir with a rubber spatula until dough is well combined.
Set in the fridge, uncovered for 30 minutes.
Preheat the oven to 350 F. Line two greased cookie sheets with parchment paper.
Use a cookie scoop to portion the dough so you have 1.5-2 tablespoons of dough per cookie.
Bake for 10 minutes, until the edges just begin to turn golden.
Right when the cookies come out of the oven, press a peanut butter cup into the center - gently so as not to break the cookie, but enough to ensure it will adhere to the cookie.