Preheat your oven to 350°F. Prepare two 8 × 8-inch baking pans.
Sift the cocoa powder and the salt into a bowl and set aside.
Place the chocolate and butter in a heatproof bowl and set it on top of a pan of simmering water, making sure the bottom doesn’t touch the water, stirring occasionally, until melted.
Mix in the sugar, remove from the heat, and set aside to cool for about 10 minutes. Once the mixture is cool to the touch, whisk in the eggs and vanilla, and then use a silicone spatula to fold in the cocoa powder and salt.
Divide the batter evenly between the two baking pans. Bake until the brownies are set, about 15 minutes.
Place the pans on a cooling rack. Allow the brownies to cool completely in the pans.
Remove the cooled brownies from the pans by turning each pan over onto a sheet of parchment paper. They will be fragile, so handle with care.
Ice Cream Layer
Wash the two baking pans. Line one of the pans with plastic wrap hanging the excess over the edge of the pan.
Empty the container of ice cream into the pan and use a silicone spatula to flatten it. Place it back in the freezer, uncovered, to harden for at least 1 hour.
Assembly
Line the second baking pan with plastic wrap, hanging the excess over the edge of the pan (this will make it easier to remove the ice-cream sandwiches from the pan later).
Place one brownie layer into the baking pan, top-side-down. Remove the pan with the ice-cream from the freezer and use the plastic wrap to manoeuvre the ice cream out of the pan and onto the top of the brownie. Discard the plastic wrap and place the second brownie on top of the ice cream. Press the top layer firmly into place, and then put the ice-cream sandwiches into the freezer overnight.
Remove the ice-cream sandwiches from the pan and cut into eight portions.
Garnish
Dip each sandwich into rainbow sprinkles before serving.
Notes
Store the ice-cream sandwiches in an airtight container in the freezer for up to 1 month.