Preheat oven to 350. Slather a heaping glob of softened butter all over a 10 inch cast iron skillet.
In the bowl of a stand mixer with your paddle attachment, beat together the butter, peanut butter, brown sugar and granulated sugar until creamy, about 2 minutes. Scrape down the sides of he bowl. Add vanilla then add eggs in, one at a time.
In a small bowl, whisk together the flour, salt and baking powder. Add the flour mixture and beat until combined. Don’t over mix it! Then add in the chips, toffee and Reeses Pieces until evenly spread through the batter.
Spoon the batter into the skillet and bake until the outside is golden but the center is soft and gooey, 35-40 minutes. After making this recipe several times, I take it out after 35 minutes for best results. It may seem a bit underdone, but it continues to set up and firm while in the skillet. Cool the pan slightly and cut into wedges. Serve with ice cream, or just add it into the skillet and mix it up that delicious magic.