Step 1: Prepare the Cookie Layers
Preheat oven to 375°F (190°C).
Line two 9×13-inch baking pans with parchment paper and lightly spray with non-stick baking spray.
Whisk together flour, cocoa powder, baking powder, and salt in a bowl.
In another bowl, beat butter and sugar until creamy.
Add the egg and vanilla extract, mixing until combined.
Gradually mix in the dry ingredients and milk, forming a soft dough.
Divide the dough evenly between the two pans. Use a spatula or food scale for accuracy.
Step 2: Bake and Cool the Cookies
Bake for 8-10 minutes or until the edges look set. The cookie should be firm but soft enough to hold the ice cream.
Remove from the oven and let cool completely on a wire rack.
Once mostly cooled, transfer to the freezer for 30 minutes to firm up before assembling.
Step 3: Assemble the Ice Cream Layers
While the cookies chill, let the chocolate and vanilla ice cream soften at room temperature.
Once softened, spread chocolate ice cream over one cookie layer.
Add a smooth, even layer of vanilla ice cream on top.
Gently press the second cookie layer on top to sandwich the ice cream.
Step 4: Freeze and Cut
Place the assembled sandwich in the freezer for at least 2 hours to fully set.
Once frozen, remove and cut into rectangular or square sandwiches.
Use a sharp knife dipped in warm water for clean cuts and wiping dry.