Let the ice cream sit at room temperature until soft, about 30 minutes. You want the ice cream to be soft not melted, otherwise the ice cream will refreeze with a gritty texture.
1.5 quart vanilla ice cream
Prepare a 13×9-inch rimmed baking sheet with a piece of parchment paper. Spread softened vanilla ice cream into the pan and spread into an even layer. Place another piece of parchment paper on top and freeze until solid. In a large bowl, whisk together flour, unsweetened cocoa powder, baking soda and salt.
140 grams all-purpose flour, 50 grams unsweetened cocoa powder, 4 grams baking soda, 3 grams salt
In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and sugar on medium speed until light and fluffy, about 2-3 minutes. Stop to scrape the sides of the bowl as needed.
170 grams unsalted butter, room temperature, 200 grams sugar
Add the eggs, one at a time, mixing until thoroughly incorporated. Beat in the vanilla extract and sourdough discard until combined.
2 large eggs, 8 grams vanilla extract, 120 grams sourdough discard
Add the flour mixture and beat on low speed until just combined. Often times, I will finish mixing in the dry ingredients with a rubber spatula.
Spread the batter onto a buttered 18×13-inch rimmed baking sheet lined with parchment paper with an offset spatula. Try your best to spread into an even layer. Preheat oven to 350 degrees F and bake for 12 to 14 minutes, or until the top of the sourdough cookie looks dry and the edges are starting to pull away from the baking sheet.
Allow the cookie layer to cool slightly in the pan, then run a small knife around the edges of the cookie. Carefully transfer the cookie with the parchment paper to a wire rack to cool completely.
Move the sourdough cookie onto a cutting board and remove the parchment paper. Cut the sourdough cookie in half, creating two 9×13-inch slabs.
Now, sandwich the frozen ice cream between the cookie slabs. Carefully wrap the Sourdough Ice Cream Sandwich completely in plastic wrap and freeze until firm, for at least 4 hours, but overnight is best!
When ready to serve, slice into 12-16 sandwiches with a warm serrated knife for clean slices. Enjoy!