Preheat your oven to 325°F. Set aside two 8-inch square metal baking pans. Do not grease or line them.
In a large bowl, place the flour blend, xanthan gum, cocoa powder, baking soda, salt and granulated sugar, and whisk to combine well.
Add the brown sugar and whisk again, working out any lumps.
Create a well in the center of the dry ingredients and add the melted butter, egg and vanilla, and mix to combine. The dough should come together and be smooth and pliable.
Divide the dough into two equal pieces. Working with one piece at a time, place it between two sheets of unbleached parchment paper.
Roll into a square that is about 8-inches all around, and about 1/4-inch thick.
Remove the top piece of parchment, and trim the edges to create a 7 1/2-inch square with a pastry or pizza wheel or a sharp knife.
Transfer the dough, still on the bottom piece of parchment paper, to one of the 8-inch square pans, folding the sides of the parchment as necessary to allow the dough to lie flat in the pan.
Repeat with the second piece of dough.
Place the two pans in the preheated oven. Bake until set in the center (about 8 minutes).
Remove the pans from the oven and allow the wafers to cool in the pans for at least 10 minutes before transferring to a wire rack to cool completely.
Make the ice cream.
In the bowl of your stand mixer fitted with the whisk attachment or a large bowl with a handheld mixer, place the heavy whipping cream. Beat on medium speed until the cream holds soft peaks.
Transfer the mixture to another bowl and place in the refrigerator to chill.
In the clean bowl of your stand mixer fitted with the whisk attachment or another large bowl with a handheld mixer, place the gelatin and 1/4 cup (2 fluid ounces) warm water.
Mix together and allow to sit until the gelatin blooms. It will swell. In a small, heavy-bottom saucepan, place the remaining 1/4 cup (2 fluid ounces) water, sugar, cream of tartar and salt, and mix to combine.
Clip a candy or deep fry thermometer to the side of the saucepan and place it over medium heat. Allow the sugar mixture to cook undisturbed until the temperature reaches the softball stage, 238°F.
Remove from the heat, add the vanilla and pour the hot sugar mixture down the side of the mixing bowl with the bloomed gelatin, making sure it doesn’t hit the whisk. The mixture will bubble and then subside.
Beat the mixture with the whisk on medium speed until the mixture is thick, white and glossy, and has nearly tripled in size.
Remove the whipped cream from the refrigerator and fold it carefully into the marshmallow mixture until well-combined.
Assemble the ice cream sandwiches.
Once the wafer cookies are cool, turn one one of the two cookie squares upside down on criss-crossed sheets of unbleached parchment paper, each sheet about 8-inches wide. Lower the paper and wafer square back into a cooled 8-inch square baking pan.
Place approximately half of the ice cream mixture on top of cookie square, and spread the ice cream into an even layer. This is easiest to do with an offset spatula.
Top with the remaining cookie square, and press down firmly but carefully.
Place the baking pan in the freezer until the ice cream is firm (at least 4 hours).
Once the ice cream is frozen, remove the whole thing from the pan, and slice in half down the center, then each half into 4 rectangles of equal size for a total of 8 ice cream sandwiches.
Wrap ice cream sandwiches individually in plastic wrap and store in the freezer until ready to serve.
Notes
For the ice cream.In place of this marshmallow ice cream, you can also use my 3-Ingredient No-Machine Vanilla Ice Cream in the same manner. For more details about the marshmallow ice cream, please click through to that post.If you would like to use a store-bought gluten free vanilla ice cream, allow it to sit at room temperature for about 30 minutes to soften before scooping it right onto the bottom wafer layer just as the recipe instructs.This recipe makes enough ice cream for double the amount of ice cream sandwiches. You can use the whole recipe of ice cream and make very thick ice cream sandwiches, or use only half, and serve the rest separately. You can also double the recipe yield to 16 ice cream sandwiches, but make the ice cream before you double the yield, or you'll always end up with twice as much ice cream as you need.