Mix the peanut butter and powdered sugar together in a small bowl until smooth and fully combined. Set aside.
Add the whipping cream, sweetened condensed milk, cocoa powder, and salt to a large mixing bowl. Use a hand or stand mixer at low speed to combine. When the cocoa is well combined, increase the speed to medium-high. Mix until the chocolate base is thick and fluffy and holds the mixer's tracks.
Spoon a layer of the chocolate base into an insulated ice cream tub or a 9 x 5-inch (23 x 13 cm) loaf tin. Use a small spoon or butter knife to lay and stretch small chunks of the peanut butter mixture across the surface. Repeat the layering process—chocolate base, then peanut butter—until both are gone.
Smooth the top with the back of a spoon or an offset spatula. If using a loaf tin, press a piece of wax paper or parchment directly onto the surface of the ice cream to help prevent ice crystals. Cover the container tightly and freeze for 8 hours, or overnight, before scooping and serving.
Notes
Mixing at low speed to begin prevents the cocoa powder from flying everywhere and helps the ingredients combine evenly before increasing the speed to high.
Insulated ice cream tubs are great for freezing and storing. I got mine at Winners and HomeSense. If using a loaf tin, press a piece of wax paper or parchment directly on the surface of the ice cream before covering to help prevent ice crystals.
Homemade ice cream is pretty hard straight out of the freezer. Let it sit on the counter for 10 minutes or so before scooping.
Storage
Cover the container tightly with a lid, wax paper, or parchment paper and freeze for at least 8 hours or overnight before scooping.