Peel and slice the bananas. Put sliced bananas in a freezer bag and place in the freezer for approx. 4 hrs.
Blend the frozen bananas, cocoa powder, peanut butter, vanilla, and salt with a hand blender until creamy.
On six of the cookies, place a scoop of the banana ice cream, then press another cookie on top to form the sandwich. Cover each with plastic wrap and place back in the freezer for approx. 1 hr.
Melt the couverture chocolate in a clean heatproof bowl over a saucepan of simmering water. Set aside to cool. Chop the walnuts and add to another bowl. Remove the ice cream sandwiches from the freezer, dunk first in the couverture chocolate, and then in the walnuts. Place ice cream sandwiches on a wire rack to set and then enjoy or put back in the freezer for later!