Preheat the oven to 350ºF. Line two 8-inch square baking pans with parchment paper, leaving extra overhang on the sides for easy removal. Spray with nonstick cooking spray.

In a medium bowl, stir the cocoa powder, espresso powder, and water together until smooth. Add 2-3 Tablespoons of the sweetened condensed milk, stirring until combined. Add the rest of the sweetened condensed milk, vanilla, and salt, and stir until completely incorporated; set aside.

In the chilled bowl of a stand mixer fitted with the whisk attachment, whip the heavy cream until stiff peaks form. Remove bowl from mixer. Add the sweetened condensed milk mixture to the whipped cream, and fold everything together until mostly combined. Fold in the chopped toffee.

Remove one brownie half from the freezer. (Leave it in the pan.) Pour ice cream on top of the brownie, and spread into an even layer. Remove the other brownie half from the freezer, remove from the pan, and peel off parchment paper. Carefully place it on top of the ice cream, pressing down slightly. Cover pan in plastic wrap and chill in the freezer until frozen solid, about 6 hours. (Overnight is best.)

Once frozen, remove pan from freezer. Run a thin knife or metal spatula along the edges of the pan to loosen ice cream sandwich slab. Using the parchment paper sides, lift the ice cream sandwich slab out of the pan and onto a cutting board. Cut into squares or rectangles, using a knife dipped in hot water. Enjoy immediately, or store ice cream sandwiches in a freezer-safe container in the freezer for up to 3 months.