Prepare your workspace: Move the oven rack to the middle position and preheat the oven to 350°F. Draw a 9-by-17-inch rectangle on parchment paper and place it ink side down on a greased baking sheet, ensuring the paper lies flat.

Mix the dry ingredients: Sift the flour and cocoa powder together into a bowl. Level off the dry ingredients using a knife for precision if measuring by volume.

Cream the wet ingredients: In the bowl of a stand mixer, combine the softened butter, brown sugar, salt, and baking soda. Use the paddle attachment to mix at low speed until moistened, then increase to medium and beat until the mixture is fluffy and light in texture, about 5 minutes.

Incorporate the eggs: Add the egg yolks one at a time while the mixer is running, allowing each to blend in fully before adding the next.

Combine wet and dry: Lower the mixer speed and gradually sprinkle in the sifted dry ingredients, alternating with the boiling water, until a smooth paste forms.

Spread and bake: Spread the batter evenly within the outlined rectangle on the parchment using an offset spatula. The layer should be very thin. Bake for about 6 minutes, until the batter puffs slightly and feels soft but set. Allow the wafer to cool completely on the sheet, then cover with foil and refrigerate until ready to use.

Line and shape: Line an 8-by-8-inch square baking pan with parchment paper, using two strips to fully cover the bottom and sides, creasing the edges to fit snugly.

Cut and assemble: Using the baking pan as a guide, cut the wafer into two 8-inch squares. Press firmly to cut through the parchment if necessary, and reserve any scraps for patching. Lay one wafer into the bottom of the pan, shiny side down, and carefully patch any cracks or gaps.

Add the ice cream layer: Once your ice cream is churned and ready, scrape it into the prepared pan over the bottom wafer. Use a flexible spatula to smooth the ice cream into an even layer.

Finish the sandwich: Place the second wafer on top, shiny side up. Press gently but firmly to ensure it adheres. Cover the pan with foil, wrap it tightly in plastic, and freeze for at least 12 hours, or until solid.

Cut and enjoy: To serve, lift the parchment flaps to remove the sandwich block from the pan. Cut into 12 equal-sized pieces, then serve immediately or store in airtight containers in the freezer for up to 1 month.