Makes 32 cookies
1/2 cup butter (1 stick), room temperature
Preheat oven to 350 F. Line 2 baking sheets with parchment paper or silicone mats and set aside.
In the bowl of an electric mixer fitted with the paddle attachment, cream together the butter and sugars until light.
In a microwave safe bowl, combine white chocolate and 2 tablespoons of peanut butter and heat in the microwave in 30 second intervals until chocolate is just beginning to melt. Stir vigorously to melt completely and fully incorporate peanut butter.
Add the melted chocolate mixture to the butter and sugars, along with remaining peanut butter. Add in egg and vanilla and mix until just combined.
Sift together flour, baking soda and salt. Add dry mixture to the wet ingredients and stir to combine. Add in mini peanut butter cups.
Scoop out 1 1/2 to 2 inch diameter balls and place on baking sheets, spacing about 1 1/2" apart. Gently press down the tops of the cookies to flatten slightly, and sprinkle with sea salt if using. Bake for 11-14 minutes, until edges just start to brown.
Allow cookies to cool for 5 minutes on the baking sheets before moving to racks to cool completely.
Store cookies in airtight containers at room temperature.