- 1/4 cup (4 tablespoons, 2 ounces, or 55 grams) unsalted butter, cold is fine
- 1/2 cup (100 grams) granulated sugar
- 1/4 teaspoon fine sea or table salt
- 3 tablespoons (45 ml) milk, any kind
- 1 large egg white
- 1/2 teaspoon vanilla extract
- 1/2 teaspoon baking powder
- 3/4 cups (100 grams) all-purpose flour
- 1/4 cup cocoa powder, any kind (see Note)
- 2 1/2 cups vanilla ice cream, or anything else you prefer
Melt butter in a large bowl halfway, then whisk it until fully melted. (This keeps the temperature down.) Whisk in sugar, salt, and milk. Whisk in egg white and vanilla; make sure the egg white is fully mixed; it can be stubborn. Add cocoa powder and baking powder and thoroughly mix. Add flour and stir just until it disappears.
Spoon into the prepared pan in small dollops and spread — an offset spatula is great here — into a thin, even layer.
Bake for 15 minutes then transfer to a cooling rack. Purely for classic sandwich aesthetics, dock all over with a skewer. Let cool completely. I transfer mine immediately to the freezer to do this quickly; it takes about 15 minutes. If your ice cream is rock-hard in the freezer, transfer it to the fridge for these 15 minutes so it will be easier to scoop.
Assemble the sandwiches: Once cold to the touch, run a knife around the cookie to loosen it from the pan and use the parchment to slide it out of the pan. Cut into half widthwise. Return the used parchment to the pan; we can use it to help shape the sandwiches. Place the first cookie half back in the pan upside-down over the parchment, and press it against the short edge. Scoop the ice cream in small spoonfuls all over and spread it evenly over. Press second cookie half, right-side-up, onto the ice cream. Use the sides of the pan and the parchment paper to help the ice cream keep its shape in the cookie and place the pan in the freezer.
Freeze for 4 to 6 hours, minimum, and ideally overnight.
To finish: Once ice cream is fully solid again, transfer your ice cream sandwich slab to a cutting board and cut it into 8 or 12 rectangles. Eat or return to the freezer right away.
Do ahead: Ice cream sandwiches keep in a freezer bag or airtight container for 3 months, although some freezers will impart a “freezery” taste sooner.