Special equipment: a waffle iron and meat mallet
For the sweet potato waffle: Preheat the oven to 200 degrees F.
Whisk together the all-purpose flour, whole-wheat pastry flour, baking powder, baking soda and 1/2 teaspoon salt in a large mixing bowl. Reserve.
Put the butter in to a small saute pan and set over medium heat. Cook the butter until foam has formed on top and subsided, and browned bits begin to appear in the bottom of the pan, about 5 minutes. Remove the pan from the heat and allow to cool slightly.
Meanwhile, whisk together the buttermilk, sweet potato puree, brown sugar, egg and egg whites in a medium mixing bowl. Once the butter has cooled slightly, add it to the sweet potato mixture and whisk until well combined.
Add the wet ingredients to the dry ingredients and stir with a wooden spoon or rubber spatula until just combined. Do not overmix. Allow the mixture to rest for 5 minutes while you preheat a waffle iron.
Preheat a waffle iron according to the manufacturer's directions. Spray the waffle iron with cooking spray. Scoop a scant 2/3 cup of the rested batter into the center of the waffle iron and press down to close the top. Cook according to the manufacturer's directions until golden brown and just cooked through, 3 to 5 minutes, depending on the waffle iron model. (Please make sure to read the directions that come with the machine.) Remove the cooked waffle onto a baking sheet set over a cooling rack and place in a warm oven. Repeat this process with the remaining batter to make 5 additional waffles.
For the pretzel-crusted chicken: Set an oven rack in the upper third of the oven and preheat it to 375 degrees F.
Remove the tenderloin from each chicken breasts and save for another future use. Cut each chicken breast in half crosswise on a large plastic cutting board. One at time, place a large piece of plastic wrap on top of a piece of chicken and use a meat mallet to gently pound the chicken until it is 1/2 inch thick. Lightly season the chicken pieces with 1 1/2 teaspoons salt and a sprinkle of black pepper.
Whisk together the all-purpose flour, 1/8 teaspoon salt and 1/8 teaspoon pepper in a shallow baking dish. Whisk together the egg whites and 1/8 teaspoon salt and 1/8 teaspoon pepper in a second shallow baking dish. Put the pretzel sticks into a re-sealable plastic bag and use a meat mallet or rolling pin to gently crush them into large crumbs. Pour the pretzel crumbs into a third shallow baking dish along with the plain bread crumbs.
In batches, coat the chicken pieces with the flour mixture, turning to dust both sides and tapping off any excess. Dip into the egg mixture, turning to coat both sides and allowing any excess to drip off. Then dip into the pretzel and breadcrumbs, making sure the crumbs coat all sides.
Heat a large skillet over medium heat with 1 tablespoon of olive oil. In batches, cook the breaded chicken until lightly browned and just cooked through, 6 to 8 minutes, adding additional oil between batches. Remove the cooked chicken to a cooling rack set over a baking sheet.
Sprinkle a generous amount of the grated Gruyere on each piece of chicken. Bake until the cheese is just melted, about 5 minutes.
For the raspberry and tomatillo jam: Put the raspberries, tomatillos, sugar and lemon juice in a small saucepan and stir to combine. Bring to a simmer over medium heat and cook until the raspberries have completely broken down, the tomatillos are softened and the mixture has reduced by at least one-third, about 15 minutes. Pour into a medium mixing bowl and allow to cool completely. The jam will thicken as it cools.
For the club sandwiches: Heat a large nonstick saute pan over medium heat and spray with cooking spray. In batches, cook the eggs until the whites are set, but the yolks are still runny, 3 to 5 minutes. If desired, use a large round cutter to cut the eggs down to the size of the chicken pieces.
Cut each hot waffle into 4 equal pieces. Top 1 piece of waffle with a few spinach leaves, 1 piece of chicken, 1 piece of waffle smeared with the raspberry and tomatillo jam on both sides, a few more spinach leaves, 1 piece of chicken, 1 egg and top with 1 piece of waffle. Sprinkle with a light dusting of confectioners' sugar. Repeat this process to make 7 additional sandwiches.