Preheat oven to 325 degrees F.
Brush 16 saltines front and back very lightly with butter.
Line an 8 1/2 by 4 1/2 inch loaf pan with foil and spray bottom with cooking spray.
In a large saucepan, melt the butter over medium heat. Reduce heat to low and stir in the sugar and cocoa powder. Stir until smooth. Remove from heat, let cool slightly, then whisk or stir in the cold egg, vanilla and salt. Lastly, stir in the flour. If you'd like to throw in a handful of chocolate chips, make sure the batter is cool so they won't melt.
Scrape the brownie batter in the loaf pan and spread evenly. Arrange 8 buttered Saltines evenly over the batter. Put the other 8 saltines on a baking sheet.
Set the loaf pan in the oven on the middle rack and the baking sheet with the crackers on the lower rack. Bake for about 25 to 27 minutes. After about 20 minutes, check the Saltines on the bottom rack to make sure they aren't too brown. They should be just slightly golden.
Let the brownie/saltine layer cool for 10 minutes, then place in the freezer and freeze until firm (or freeze and wrap until ready to use).
To assemble the sandwiches, line the loaf pan you used to make the brownies with plastic wrap. Tip: Try wetting the pan a little before adding the plastic wrap. It helps go on smoother. Spread some softened ice cream (as thick as you want, so amount varies) across the pan. Cover and freeze for about 2 hours or until very solid.
Take the frozen brownie/cracker out of the freezer and set it cracker side down on a flat surface. Unwrap the frozen ice cream rectangle and set it on top. Arrange reserved 8 toasted Saltines over ice cream, then wrap and freeze again until solid.
Remove from loaf pan and cut into 8 squares. Store squares in a freezer bag.