Instructions

To make the ice cream filling:

1. Pour the cream into a small heavy bottomed saucepan and bring just to a boil. Meanwhile, place the white chocolate in a medium heat proof bowl. When the cream boils, remove the heat and pour over the chopped chocolate, and the vanilla and salt, and whisk until the chocolate is melted and the mixture is smooth. Transfer to an airtight container and refrigerate overnight.

To make the cookies:

1. Preheat the oven to 350°F. Spray a 13"x18"half-sheet pan with nonstick spray (or butter it) and line with parchment paper. (The spray is to help keep the paper in place when you spread the cookie batter.)
2. Add the flour, cocoa powder, baking powder, and salt to a medium bowl and whisk together.
3. Add the butter, brown sugar and granulated sugar to a large bowl and mix until well combined. Add the eggs, one at a time, and mix until smooth. Add the vanilla extract and stir again. Add the flour mixture and stir until combined. The batter will be thick (like most cookie batter).
4. Transfer the batter to the prepared pan and smooth into an even layer, leaving a narrow border of parchment paper around the edges, with an offset spatula. (It will be kind of pain to spread, but don't worry, that's the hardest part of this recipe.) Bake until the cookie has set and looks dry on top, about 10-12 minutes. Let cool completely, then wrap in plastic wrap and freeze for at least an hour or overnight.
5. Invert the chilled cookie to a cutting board and peel off the parchment paper. Slice in half and transfer back to the sheet pan. Remove the whipped cream mixture from the refrigerator and whip with an electric mixer until it holds stiff peaks. Spread in an even layer over one of the cookie halves. (Or use softened ice cream.) Gently place the other cookie half on top and freeze until firm, at least 8 hours or overnight. 
6. Remove from the freezer, trim off the edges to even them out, then slice into 12 even rectangles. Serve immediately or wrap individually in plastic wrap (or slip into sandwich bags) and freeze for up 2 weeks.