Preheat the oven to 325° F. Coat a 13x9x2 inch pan with nonstick cooking spray.
PREPARE THE CAKE:
In a large bowl, whisk together the oat flour, dry milk powder, sugar, cocoa, baking powder, baking soda, cinnamon and salt. In a separate bowl, whisk together the applesauce, egg whites and vanilla.
Stir the wet ingredients into the dry ingredients and mix just until combined. Don't over-mix or you may toughen the texture of the cake.
Pour the batter into the prepared pan, smoothing to the edges, and bake until springy to the touch and a toothpick inserted into the center of the cake comes out clean- it should take anywhere from 25 to 35 minutes. Check at 25 and add minutes onto baking time if necessary.
Cool cake to room temperature on a wire rack.
PREPARE THE FROSTING:
In a medium bowl, use an electric mixer at low speed to beat the cream cheese, sifted powdered sugar, cocoa and vanilla in a medium bowl until smooth. Slowly add the nonfat milk and beat until it's a good spreading consistency, about 1 minute.
Swirl frosting over top of cake. Cut into 15 squares, and serve.
Notes
To make your own oat flour, grind 1¼ cups old fashioned oats in a coffee grinder or strong blender.
Substitutes for oat flour: try using rice or barley flour. Almond flour can be used too, but the flavor will be different and it will have a higher fat content.
I baked this cake with regular sugar, but you can certainly bake with a sugar replacement, if you'd like. Just follow instructions on the bag for how to replace the sugar. I usually like the Swerve brand granular when I'm replacing the sugar. It's a cup-for-cup replacement.
For the frosting, Swerve also sells a powdered version if you want to try that to be sugar free.
I made this cake in a 9x13-inch pan, but I think it would work just as well baked in two 8-inch round cake pans. There certainly is enough frosting to use for a two layer cake.