Line a 17 x 11 inch baking pan. Preheat the oven to 350°F/ 180°C. (You can use a half sheet pan, but then brownie will come out slightly thinner).
In a heatproof glass bowl, combine the salt, chocolate and butter. Heat in the microwave at 1 minute intervals, until the butter and chocolate have melted and it's smooth. You could melt these over a pot of simmering water as well (double boiler).
Set aside until it is has cooled down a little and is warm to the touch. Add the sugar and vanilla and using a hand mixer, mix until well combined.
Add the eggs, one at a time, mixing well between each egg. Mix in the flour by hand, or our hand-held mixer at low speed.
Spread the mix evenly on the lined baking tray.
Peanut butter swirl
Microwave the peanut butter for a few seconds until it has softened slightly (it should not be hot or liquidy).
Add the powdered sugar and salt and mix well until smooth.
Place dollops of the peanut butter mix on the brownie batter and swirl them into the brownie mix using a flat spatula or knife.
Bake it in the preheated oven for 15-20 minutes, until a clean toothpick inserted into the middle of the brownie comes out with only a few crumbs attached (if your toothpick comes out completely clean, then it might be a little dry, but it'll still be great since it's going to be used for ice cream sandwiches).
Remove from the oven and let it cool. Gently loosen the brownie from the pan and let it cool in the freezer till it's hard and easy to cut (few hours at least).
Ice Cream Sandwiches
Soften the ice cream slightly for a few minutes. Cut the brownie sheet in half and place scoops of ice cream on one half. Spread the ice cream over the surface, leaving about 1 inch of space along the edges. Place the other brownie half on top and press it down gently. Return to the freezer to harden (few hours or overnight).
Cut the edges off. To get 12 bars, cut this in half lengthwise, and then each half into 6 bars (about 1.5 inches wide). Or you can cut them into whatever size you prefer!
Serve immediately, or return them to the freezer for a few hours, if you want to dip them in chocolate.
Make the chocolate magic shell, and dip one half of each cold brownie ice cream sandwich and let it set and harden on a parchment paper.