Preheat oven to 350°F. Spray a regular sized muffin tins with cooking spray.
Whisk together flour, baking soda, and salt, set aside.
Beat butter and peanut butter until smooth. Add sugars and beat on med-high until pale and fluffy (approx. 2-3mins).
Reduce speed and add egg and vanilla. Beat until combined.
Add flour mixture and mix until just combined.
Using a large cookie scoop (3 Tbsp), scoop dough into muffin tins, press down to flatten slightly.
Bake for approx. 12 mins or until lightly browned and mostly set.
Remove from oven and immediately use a small jar or container to press firmly down in the center to create a well.
Cool in pans for 10 mins. Twist to release and place on wire rack to cool completely.
Peanut Butter Filling:
Beat butter until creamy. Add peanut butter and beat until well combined (2-3 mins).
Add sifted powdered sugar in one 1/2 cup at a time and beat until combined. Add vanilla.
Add cream one Tbsp at a time until desired consistency. Beat on high for 3 minutes until fluffy.
Place water, sugar, and corn syrup in a medium saucepan. Stir to combine.
Insert a candy thermometer into the pot and heat over medium-high. Do not stir from this point on as crystals will form.
Ensure mixer bowl and whisk are completely grease free.*
Place egg whites and cream of tartar in the bowl of a stand mixer.
When the sugar syrup reaches about 225°F, start whipping the egg whites to soft peaks. Approx 3-4mins.
When the whites are ready, the sugar syrup should be at 240°F. Remove from the heat, turn mixer to medium and very slowly and carefully pour the sugar syrup into the whites in a thin, steady stream.**
Once all of the syrup is in, set mixer to medium/high and continue whipping. The whites will deflate at first, but they will thicken and fluff up.
Continue to whip for 7-8 minutes, or until the mixture is thick and glossy. Add in vanilla and whip until the fluff has cooled.
Pipe peanut butter filling into each cup (about 2/3 of the way up).
Pipe a dollop of marshmallow fluff on top and sprinkle with chopped peanuts if desired.
Serve at room temperature and eat within 2-3 days. Or freeze for up to 4 weeks.
*If there is even a speck of grease (or egg yolk) the meringue will not whip. I wipe everything down with lemon juice before adding the egg whites. **The sweet spot for this is getting the stream right between the whisk and the sides of the bowl, so the syrup doesn't get splashed by the whisk.You may have some marshmallow fluff left over which you can store in an airtight container in the freezer and use it at a later date for S'mores Macarons, S'mores Baked Alaska, or good old Rice Krispie Treats.