Adjust oven rack to middle position and preheat to 350F/180C. Lightly spray 17½ x 12-inch half-sheet pan with nonstick cooking spray and line with parchment paper. Set aside

Over a medium bowl sift together the flour, cocoa, and baking soda and salt.

In 30 second increments melt together the butter and chopped chocolate. This should take no more than a minute and a half.

Add the sugar to the melted chocolate and butter mixture and whisk until the sugar has dissolved and the mixture is cooled down. Lastly, stir in the vanilla and eggs until combined.

Mix the wet ingredients into the dry to form a thick rich batter similar to a brownie.

Transfer the batter into prepared baking sheet, spread batter evenly in pan using and offset metal spatula.

Bake until cookie springs back when touched with your finger, about 10 to 12 minutes.

Cool in pan on wire rack for 5 minutes, then run paring knife around perimeter of baking sheet to loosen. Invert cookie onto work surface or large cutting board; carefully peel off parchment. Cool to room temperature, about 30 minutes.

After the cookie layer is done cooling, measure, score, and cut at the halfway point. The cookie has a tendency to stick after cooling, so use a bench scraper to lift it from the cutting board onto a large piece of plastic wrap. Empty the ice cream onto the cookie, and using your clean or gloved hands, squish the ice cream to distribute evenly. Place the second cookie layer on top of the ice cream and wrap in plastic wrap. Place on a cookie sheet and freeze until the ice cream hardens again, about 1-2 hours.

Cut the bars into thirds and then each third into four pieces to make 12 ice cream sandwiches. Serve immediately or wrap each ice cream sandwich in plastic wrap and return to the freezer.