everybody wins

The final tweaks are in, and the additional egg white had the desired "glue" effect. Here's the recipe I used for the test run, since we had quite enough lemon squares already:

Low Carb Cinnamon Shortbread
16 squares

1/2 C almond meal
1/2 C Designer Whey French vanilla protein powder
1/2 tsp xanthan gum
1/4 C Splenda pourable
2 tsp cinnamon
1/8 tsp nutmeg
1/2 C (1 stick) unsalted butter, softened
1 egg white

Preheat the oven to 325 degrees.
Line an 8x8 baking pan with foil, and spray the bottom of the foil with no-stick cooking spray.
Put the almond meal, protein powder, xanthan gum, Splenda, cinnamon, and nutmeg into the workbowl of your food processor fitted with the "S" blade. Process to combine everything, about 15 seconds.
Cut the butter up into rough chunks and add to the food processor bowl. Pour in the egg white. Process until the mixture forms a smooth, sticky dough, and no lumps of butter are visible.
Scrape the dough out into the pan, and spread it evenly to the sides and corners. This dough is somewhat self-levelling, but do take care that there aren't any thin spots.
Bake in a 325 degree oven for 25-30 minutes, until just starting to brown around the edges. Do not underbake or the texture will be gluey in the middle.
Allow to cool about 5 minutes in the pan, then, using the edges of the foil, lift the shortbread out of the pan onto a cooling rack. Allow to cool completely before cutting. Makes 16 good size squares.

You can use double the ingredients and bake in a 9X13 pan for excellent results. You may want to dial back the cinnamon to 3+1/2 tsp, though. These are super-cinnamon-y; you could probably decrease the cinnamon to just 1 tsp if you're concerned about cinnamon overload.

Approximate nutrition information, per square: 81 calories, 7 g fat, 1.5 g carbohydrate, 0.6 g fiber, 3 g protein

If you want to make the high-carb version of this, use regular flour in place of the protein powder and almond meal, and sugar in place of the Splenda; omit the xanthan gum and egg white. High carb baking is so easy.