Heat your oven to 350°F (175°C).
Prepare pans by lining the bottom and sides with foil and lightly coating with nonstick spray. Arrange crackers in a single layer.
Make toffee by melting butter and brown sugar in a saucepan, then stir in vanilla. Pour over cracker-lined pan and spread evenly.
Bake for 10 to 15 minutes, then scatter chocolate chips over the caramel crackers, wait for them to soften, and spread into an even layer. Sprinkle with nuts and sea salt.
Chill crackers in the freezer until cold and solid. Cut into halves if made in one pan.
Assemble ice cream sandwich block by layering candy sheets with ice cream in between. Return to the freezer for several hours until solid.
Cut block into individual sandwiches using a sharp serrated knife. Return sandwiches to the freezer to firm up before storing or serving.