Preheat oven to 350 degrees. Grease a cast iron skillet with 1 tablespoon butter.
In a large bowl whisk together melted butter and sugar, until combined. Add the eggs, vanilla, and salt, whisk until evenly combined. Then, fold in cocoa powder, flour, and baking powder, just until combined. Next, fold in chocolate chips.
In the prepared pan, pour the batter. Next, add the peanut butter by dolloping 9 tablespoons on top of the brownie batter. With a butter knife, create a swirl effect gently through the brownie mix with the peanut butter.
Place in the oven and bake for 30 to 35 minutes, or until the center is set. Remove from the oven. Allow to cool.
Make the ice cream
In a quart size resealable bag, add half and half, milk, sugar, and vanilla. Seal tightly. Set aside.
In a gallon size bag, add 2 cups of ice, and then salt. Place the filled quart size bag into the gallon bag. Then, seal gallon bag. Shake gallon bag vigorously for 20 minutes. Ice cream will have a soft serve consistency. Place in freezer until ready to use.
Once the ice cream is ready, add on top of the brownie. To make it even tastier, the brownie is best served warm. Serve, and enjoy!