Make a soft cookie
Position a rack on the center rung of the oven. Heat the oven to 350°F. Lightly grease the bottom of an 18x13-inch rimmed baking sheet. Line the pan with parchment to cover the bottom and the edges of the pan’s longer sides. Combine the flour, cocoa, baking soda, and salt in a medium bowl; whisk to blend. In a large bowl, beat the butter and sugar with a hand-held electric mixer on medium high until well blended and lightened in color, about 3 minutes. Beat in the vanilla. Add about a third of the flour mixture and beat on medium low until just blended. Pour in half the milk and beat until just blended. Add another third of the flour and blend. Pour in the remaining milk and blend, and then beat in the remaining flour.
Distribute the dough evenly over the prepared pan in small dollops. Using one hand to anchor the parchment, spread the dough with a spoon or spatula. Drag a rectangular offset spatula (I like my burger flipper) over the dough to smooth it into an even layer, rotating the pan as you work. Brush or spray a sheet of parchment the same size as the pan with oil, and lay it, oiled side down, on the dough. Roll a straight rolling pin or a straight-sided wine bottle over the paper (or swipe it with a dough scraper) to level the batter. Carefully peel away the parchment. Bake until a pick inserted in the center comes out clean, 10 to 12 minutes. Set the pan on a wire rack and let cool to room temperature.
Layer the cookie with ice cream, then chill
Lay two long pieces of plastic wrap in a cross shape on a baking sheet. Slide a knife along the inside edge of the pan containing the cookie to loosen it. Invert the cookie onto a large cutting board. Peel off the parchment. Using a ruler as a guide, cut the cookie crosswise into two equal pieces. Place one layer, top side down, in the middle of the plastic wrap (a wide, sturdy spatula will help the transfer).
Remove the ice cream from the freezer and take off the lid. It’s important to work quickly from this point on. (If the ice cream gets too soft, pop it onto a plate and back into the freezer to harden up.) Using scissors or a sharp knife, cut the container lengthwise in two places and tear away the container, as shown below.
Set the ice cream on its side. Cut the ice cream into even slices, 1/2 to 3/4 inch thick, and arrange them on top of the cookie layer in the pan, pairing the smallest piece next to the largest. Using a rubber spatula, gently yet firmly smear the ice cream to spread it evenly, as shown below. (It helps to put a piece of plastic wrap on the ice cream and smear with your hands; remove the plastic before proceeding.)
Position the remaining cookie layer, top side up, over the ice cream, as shown below. Press gently to spread the ice cream to the edges. Put a clean piece of plastic on top and wrap the long ends of the bottom sheet of plastic up and over the cookie layers and ice cream. Put the baking sheet in the freezer and chill until the sandwich is hard, about four hours and up to two days.
Cut and garnish the sandwiches:
Take the baking sheet out of the freezer. Lift the package from the pan, transfer it to a cutting board, and line the pan with a fresh piece of plastic. Peel the top layer of plastic off the sandwich (you can leave on the bottom layer.)
Working quickly, use a ruler and a long, sharp chef’s knife to score the cookie, dividing it into twelve: three across the the short side and four across the long side. Cut the sandwiches as shown below, wiping the blade clean as needed. (If your kitchen is very warm, put the pieces back into the freezer to firm, or work with one strip at a time, keeping the rest in the freezer).
Garnish the sandwiches, if you like: Fill a small, shallow bowl with your chosen garnish and set it next to your work surface. Press some of the garnish onto some or all of the sides of the sandwich. Set the sandwiches back on the baking sheet and return to the freezer immediately. (If your kitchen is warm, keep the sandwiches in the freezer and garnish one at a time.) Once the sandwiches are hard, wrap them individually in plastic and store in the freezer.
Make Ahead Tips
Individually wrapped sandwiches will keep for up to two weeks in the freezer.
Chocolate-mint cookie: add 1/2 tsp. peppermint extract when you add the vanilla.
Chocolate-orange cookie: Add 1/2 tsp. natural orange flavor or orange extract when you add the vanilla.
Chocolate-espresso cookie: Mix in 1 level Tbs. instant coffee granules when you add the vanilla.
Chocolate-ginger cookie: Add 1/2 tsp. ground ginger to the dry ingredients.
nutrition information (per serving):
per sandwich without garnishes;
Photo: Scott Phillips