Preheat oven to 350°F Coat two 13x8 inch rimmed baking sheets with cooking spray, line with parchment, and spray parchment.
Whisk flour, baking powder and salt in a medium bowl. Melt butter and chocolate in a large heatproof bowl set over a pot of simmering water, stirring until smooth, Remove from heat, add sugar and stir until dissolved. Stir in eggs 1 at a time, and then stir in vanilla. Stir in flour mixture until just incorporated, do not overmix.
Divide batter between pans. Use an offset spatula to spread batter into thin, even layers. Bake, rotating pans once, until batter just begins to set, 8-10 minutes. The surface will look dry, and testers inserted in the center wil come out wet. Let cool completely in pans on wire rack.
To assemble, place the flat side of a lare wire rack over 1 brownie, and invert rack, brownie, and pan together. Remove pan and parchment. Repeat with remaining brownie. Place a clean rimmed baking sheet over 1 brownie, and invert pan, brownie, and rack together. remove rack. The brownie should be in the pan, top side down.
Spread ice cream in a smooth, even layer over brownie in pan. Align pan lengthwise with remaining brownie on rack, and use rack to quickly and carefully flip brownie onto ice cream. Remove rack. Press top brownie gently. Cover and freeze until firm, at least overnight, or up to 1 week.
Uncover and cut into squares. serve immediately or cover and freeze ice cream sandwiches overnight.
Peanut Butter Ice Cream: Heat cream, milk, and peanuts in a medium saucepan over medium heat until bubbles begin to form around the edges. Remove from heat, cover, and let steep at room temperature for 3 hours, or refrigerate overnight.
Prepare an ice-water bath. Uncover cream mixture, and reheat until hot but not boiling. Whisk egg yolks, sugar and salt in a large bowl. Add cream mixture in a slow steady stream, whisking constantly until well combined. Return mixture to saucepan, and stir constantly over medium heat until thick enough to coat the back of a spoon, about 8 minutes. Do not let the mixture come to a simmer.
Strain through a fine sieve into a bowl. Discard solids. Set the bowl in the ice-water bath. Stir in vanilla, and let cook, stirring occasionally, until cold, about 20 minutes.
Freeze mixture in a ice-cream maker according to manufacturers directions. Transfer to a large bowl, and fold in peanut butter until ice cream is swirled. Use immediately or freeze for up to 1 week; soften in the refrigerator before use, about 10 minutes.