Step 1

Let the ice cream sit at room temperature until soft, about 30 minutes. Line a 13x9-inch baking pan with parchment paper. Spread the softened ice cream into the pan in an even layer and freeze until solid.

Step 2

Preheat the oven to 350F. Butter a large 18x13-inch baking sheet and line with parchment paper.

Step 3

In a medium bowl, whisk together the flour, cocoa, baking powder, and salt.

Step 4

In a large mixing bowl or the bowl of a stand mixer fitted with the paddle attachment, beat the butter and sugar on medium speed until light and fluffy, about 3 minutes. Stop and scrape down the sides of the bowl as needed. Beat in the eggs, one at a time, until evenly incorporated. Beat in the vanilla.

Step 5

Add the flour mixture and beat on low speed until fully combined, stopping to scrape down the bowl once during mixing. Using an offset spatula, spread batter into the prepared pan in an even layer.

Step 6

Bake for 10 to 12 minutes or until the top appears dry, and sides start to pull from the pan. Poke the top with fork tines, if desired. Let cool slightly in the pan. Run a knife or a small offset spatula around the edges of the cookie. Transfer the cookie with the parchment paper to a wire rack to cool completely.

Step 7

Transfer the cookie onto a cutting board and remove the parchment paper. Cut the cookie widthwise in half, creating two 9x13-inch slabs. Sandwich the ice cream slab between the cookie slabs, trimming the sides as needed to fit evenly. Wrap in plastic wrap and freeze until firm, 4 hours or up to overnight. Cut into 12 sandwiches using a warm serrated knife when ready to serve.